Okay, lots of topics to address.
The Gatekeeper

Before Molly's phone call, Harry was going to use LC to investigate black magic. Without the Gatekeeper's message, Harry wouldn't have been planning on using LC to track black magic. Instead, he would have been looking into the Eb's request or investigating the accident. That said, I think there is too much evidence the Gatekeeper doesn't want Harry dead and that the Gatekeeper could easily kill Harry and make him disappear to believe that the Gatekeeper is trying to get Harry to kill himself.

The significance of the accident

JB is a lazy writer and doesn't introduce events that don't have a significance. Granted the significance might be character development rather than plot development, but there really isn't much character development in the fact that Harry can be the victim of random accidents, so I doubt that is the case. Now, the accident was the means JB used to prompt the Harry/Murphy conversation, and it's purpose could have been limited to setting up that conversation. However, like Uriel, JB likes to kill two birds with one stone, so I think the accident will have later significance. Specifically, I think Harry will discover the attempted murder was a moment-of-opportunity attack by someone who was present in Chicago for the execution and saw an opportunity to possibly remove Harry as a problem.

The interrupting phone call

This event is entirely separate from the accident. I reject the theory that the accident was designed to delay Harry so the phone call would be in time to save him. First, as indicated by the cop's comments, the accident was far too serious to be a benign attempt to save Harry. Second, there was far too much time between the accident and the phone call and the phone call was far too close to the start of the ritual for anyone to have planned it so precisely. If anyone could accurately predict the exact time that Harry would use LC, it makes far more sense that they would manipulate the timing of the interrupting phone call than trying to delay the use of LC until after the call. In fact, don't we find out that Molly was prompted/manipulated into calling Harry? Given Lea's backdoor into Harry's apartment, someone like Mab would be able to monitor Harry's use of LC and convince Molly to call at the right time to stop it. Alternatively, someone time traveling might have knowledge of the exact time when Harry would try to use LC and convince Molly to call at the right time to stop it. It's much easier than setting up an accident and hoping it causes a long enough delay to stop Harry from using LC while not injuring Harry too severely.

Lash and LC
Adding to the lack of time argument, here is the following:

Lash spent considerable effort to convince Harry not to use LC. If she had known about the specific flaw, her next step would have been to use that as a bargaining chip. Once Harry had proven he was going to use LC at any cost, she would have told him she knew of a specific flaw and warned him that he would never be able to fix it in time to save Molly. She then would have offered to identify the flaw and taught him to fix it, either as part of a bargain (maybe an agreement that he would sit and talk with her for 30 minutes) or as a display of her good will (i.e. making Harry more dependent upon her knowledge). There is no way she has Harry secretly fix it, losing whatever advantage she could have gotten from helping him fix it.

Was LC destroyed

Personally, I think yes, but given JB's vague answer, that's really just a guess. However, it's important not to cut off the rest of JB's answer to that question:

It was made of (mostly) pewter. The rest was plastic. Harry hadn’t taken steps to make it less destructible (which would have interfered with its function anyway–it was built to be sensitive, not tough). There was just no way it could have survived the fire. And no, the FBI didn’t confiscate it.
Changes is, in many ways, about loss. About encountering it and feeling its pain. That happens to all of us, sooner or later. There’s no avoiding it.
The real question is, how do you pick up the pieces and keep going, afterward.

I think the bolded part of that quote provides strong evidence that if LC is still around, Harry won't be getting it back. Instead, it will be used to complicate Harry's life, either because it is a powerful tool in the hands of an enemy or because there might be some way to track its creation back to him, which would be bad if the Wouncil found it, or both.

Oh I saw this coming after reading your first paragraph!  Awesome idea, I'll add it as a #4, although I feel as though this idea doesn't exclude it being a time travel, or alternate universe Harry.

Maybe I worded that part poorly. I actually agree this explanation makes it more likely it was time travel. Look at the car explanation:

Harry's car is stolen. Someone in the future warns Harry so he moves the car and it isn't stolen. The problem is the new future, the car isn't stolen, so Harry isn't warned, which means he doesn't move the car, which means it is stolen, which means he is warned, which means.... and it repeats forever.

This is the same problem that applies to LC. If LC blows up, causing someone to go back in time to fix LC, then LC doesn't blow up in the new future and nobody will go back in time, etc...

Now look at this chain of events:

1. Harry's car is stolen
2. An earthquake causes the parking garage where Harry's car is parked to collapse.
3. Future Harry wants to save his car from the Earthquake and goes back in time and moves it.
4. In the new future, Harry's car isn't stolen, but the Earthquake still happens. Thus, Harry still knows he needs to move the car and as a side effect, he gets to prevent his car from being stolen without creating a paradox.

Applying that to LC we get the same result.

1. LC blows up
2. Some other chain of events create a situation where someone needs to use LC and doesn't have enough time to build one.
3. A person goes back in time to use LC, which requires them to first fix the glitch.
4. In the new future, LC no longer blows up, but as long as the series of events in #2 still occur, then the time traveler still learns of the need to travel and gets to fix LC without creating the paradox.

True enough, excpet Carmilla was first. Dracula actually copied it, in many respects. ( in RL, Stoker mugged it like a small kid with a giant bag of candy.)

That's my point. Maybe some normal human wrote Carmilla based upon their perceptions, which gave the White Court vamps an idea to publish a more accurate how-to guide.

and the second problem is that what we know of the Black Court doesn't ,match Dracula worth a darn, either. Frankly, it matches Lovecraft much better.

... hmm, well, maybe the White Court didn't want to comletely destroy the Blacks so they published a less accruate version? Heck, maybe they published Carmilla and humans were too scared to follow its instructions, so they tried again with Dracula, making the vamps appear weaker so humans would give it a go.

No guard duty, no power.  And Mouse cheats by guarding something possibly just as metaphysically important...  Harry.  I think their mutual loyalty is heavily tied into this too.

Makes sense. Just look, everytime Mouse does something powerfu, he's guarding something. Barks to wake up the inhabitants of a burning building, survives getting hit by a car in defense of Harry, expresses his willingness to take on Mab's #2 Faerie to protect Harry and his allies.

Um, beg pardon, but Harry tried all those spells. they were being blocked by Shaggy. LC might have worked , as it was a demi artifact designed right for that purpose, that Shaggy did not know about.

etc, etc.. TC , starting pg 211

Shaggy was blocking all the stadard spells 

I agree Harry would have at least tried LC, because he was desperate (it is very suspicious Harry doesn't at least mention LC). But, I think less-emotionally involved individuals, like Bob and Mab recognize that LC would not have helped find Thomas.

However, from the discussion in PG, I don't think LC was designed in a manner which, under the specific circumstances of TC, would help find Thomas. As evidenced by the text you quoted, Harry's problem was that the sibling link wasn't strong enough. Using LC would still have been dependent on a sibling link, so it too, would have been insufficient. Bob would recognize that and would therefore not recommend using LC, because Bob would understand that LC would not succeed where the normal tracking spells woufailed.

What we think we see in PG appear to be "nudges" done with great care.

This is a huge problem I have with the Harry time-travel theories as expressed.

First, <Watson Hat> Harry (or anyone else for that matter) going back in time for the purpose of preventing Harry from being injured is not nudging, it is direct action that changes the past. It would be the equivalent of the Gatekeeper going back in time to stop Molly from using black magic in the first place, rather than having Eb warn Harry that there is black magic being used in Chicago. It would be the equivalent of Harry saving the two girls at Splattercon! or going back in time to prevent Maggie Jr. from being kidnapped.

Second, <Doyle Hat> a story written for the exclusive purpose of explaining something in a prior book is lame. The explanation of what happened in the past book has to be relevant to the successful conclusion of the current book. Thus, no future book is going to tell the story of Harry going back in time to save himself, solely for the purpose of explaining the events of a past book.

There is a solution to both of these problems, fixing LC cannot be the purpose of any future time travel, it has to be something that happens incidental to the true goal of the time travel. The story would go as follows: in Book ##, Harry needs to accomplish task "x". In order to do so successfully, he needs LC, but as we all know, LC was destroyed in Changes. Thus, Harry goes back in time for the purpose of using LC to get the information he needs to accomplish task "x" in the future. He shows up at the apartment, heads down to the basement to use LC and sees a huge gaping flaw. He fixes it, chuckles when he realizes he just created and solved his own mystery, and proceeds to use LC for whatever it is he needed it for, then returns to the present time of Book ##. This also explains why Thomas is freaked out in PG (thanks Serack). Either Thomas is there when Harry shows up and finds future Harry disturbing or, Thomas thinks Harry is already home when PG Harry shows up at the door, and can't figure out how Harry snuck out of the basement without him hearing.

Now, here's my problem <Doyle Hat> with the above story solution: It's boring. JB could write a compelling story about Harry going back in time to use LC to accomplish future task "x" and completely freaking the crap out of Thomas, but throwing in - I also saved my own life - is boring and parallel to a major storyline in the Harrry Potter books. 

The only solution that solves both the "no direct action" and "it's boring" problems is for LC to have been fixed by someone other than a time traveler.

2) As such, the only thing you can actually accomplish with time travel is something that already happened.  If you kill someone in the past, then historically (your timeline), he must have been killed by you as well.

That argument is completely circular. How do we know future Harry can fix LC - because he already did. How do we know future Harry can't go back in time and make Molly his apprentice - because we already know he didn't. If LC was already fixed in the past, then there is no need for future Harry to time travel to fix it.

If Harry can go back in time and instruct Bob to mention to past Harry that LC was fixed for the purpose of causing Harry to go back in time to fix LC, then why can't future go back in time and instruct Bob to start up a discussion about that time he and Susan got dirty and start wondering if half-vamps can still get pregnant, causing Harry to give Susan a call and find out about Maggie Jr. long before the events of Changes. Under your theory, the only reason Harry can't do that is because he didn't do that, but the only reason we know he didn't do that is because JB chose to write it that way.

The whisper didn't change the route, it just added the step of calling Kincaid.

Whisper thought: I'll call Uriel first, but he won't be able to help, so I'll call Mab and turn into a monster. I need to make sure I am stopped before that happens. Calls Molly in to help.

After-whisper thought: I'll call Uriel first, but he won't be able to help, so I'll call Mab and turn into a monster. Calls Uriel.

Harry wasn't going to skip calling Uriel, he was just being realistic because he knew Uriel wouldn't help. And if by some miracle, Uriel did help, when Molly found out he wasn't the WK, she would have given the memory back so Harry could call off the hit. In fact, the hit was on the new WK, so if Harry didn't become the WK, Kincaid wouldn't have shot him (it's not unreasonable to think Kincaid would have a way of learning whether Harry actually became the WK).

When you look at it like that, it still sounds elegant. It also sounds slow as molasses. If the Fallen is only allowed to offer some sort of boost and has to wait until after the offer is taken before they're allowed to actually tempt the person, how the heck is their success rate usually so good that they get a person to pick up the coin in a few weeks at most?

I don't think most bearers take that first step of rejection. They probably slip the coin in their pocket and carry it around like a secret treasure. Without that first rejection, they would be open to the full temptation at the start.

On the gripping hand, this does align with the idea that forcing someone to pick up a coin won't work, because even after you are forced to pick the coin, you still have to accept the first offer of power without either the Fallen or the Fallen's shadow being allowed to tempt you. Unless, of course, they cheat.

No, I think once you touch the coin the Fallen can try to tempt you. It is only following an act of refusal (Harry sealing the coin) that the Fallen is forced to resort to more subtle means.

How do you know you won't like it if you don't give it a taste first?

Classic temptation, classic folly. I don't need to taste peach pie (which I would probably like) to know it is unhealthy and I don't need to steal a sports car to know it is morally wrong.