Honestly, I always thought that method of IDing the # of necromancers was shoddy.  I think too much emphasis was put on it in the blurb, and never really thought it was worth this much discussion.

You're probably right.  The main reason I always found that part interesting is because given that Jim is not one for extraneous details, you have to figure that he mapped out exactly what each of those necromancers did at each of those locations and had a reason for all of them.  Not to mention Lasciel's shadow screwing around with Harry's head the whole book induces a certain amount of paranoia.  I have never been able to shake the feeling that something weird was going on in Dead Beat which Harry wasn't catching on to.  Of course I get that feeling about most of the books.   

It's also interesting because this meeting in DB is also where the Mort's haircut discrepancies arose that Serack mentioned earlier.  I bet someone could turn this into an awesome conspiracy theory.

3) The Hob attack

The Hob attack is another weird event.  On the face of it, it seems like an attempt to abduct or kill the Archive, but in fact, I find it hard to believe that gang of Hobs would be a reasonable threat to her.  This is after all a being that is at least as powerful as one of the Faerie Ladies (according to the underestimation of the Warden files), and we see her hold off a horde of Denarians without much trouble.  Add to this Kincaid and Luccio, (and possibly also Harry and Michael - Mab must be keeping tabs on her Emissary), and using a bunch of Hobs for such an operation seems downright stupid -- which Mab is not.  At the very last resort, Ivy could retreat into the Nevernever...

Thus, I suggest that if Mab really wanted to kill the Archive, she could (and would) have easily assembled a way more powerful force (e.g. sending Santa to abduct the child Archive would have been a cool twist), one that would have a better chance at overpowering the Archive and her protectors..

So if the Hob attack was not meant to kill the Archive, what was its purpose?

One possible answer is that all this is simply "warning shot" for the Archive.  Sending a bunch of Hobs to attack her is essentially Mab's way of telling her "Someone is up to no good. Take extra precautions."  One could even say that those clothes thrown around the aquarium to obscure Ivy's smell is a precuation taken by Kincaid as a direct result of this obscure warning. 

Actually I have been thinking about this attack a lot and I am not sure that the conclusion that the Hobbs were not much of a threat to Ivy is reasonable.  Think about what we know about Ivy in a fight from the Shedd Aquarium.
1. Her first tactic is to veil herself and wait for Kinkaid to kill the attacker(s).
2. While she has incredible skill with magic she still a little girl and probably doesn't have the stamina to fight for a long time even though she is really efficient with her magic.

Now consider the abilities of the Hobbs:
1. They make the area pitch black and fight without seeing (as they have no eyes).  As such not only does the myrk reduce the effectiveness of Kinkaid and Luccio but Ivy can't hide with a veil because invisibility is pretty much useless against an opponent with no eyes.
2. There were a lot of them.  A LOT!  Had Harry not dispelled the Myrk and Michael and Tiny cleared some of the Hobbs out the fight might have gone much much longer.

Also remember that Mab removed Harry's knowledge of fire evocation.  Harry used fire to turn on the sprinklers to dispel the myrk.  It could be that Mab miscalculated by not expecting Harry to think of using thaumaturgy to start the sprinklers when she only removed his fire evocation knowledge.

Now that doesn't mean that the Hobbs would necessarily have won but it does indicate that they were a particularly effective threat against the Archive even though they were not necessarily Mab's strongest troops in a general sense.  Also even if Mab didn't think the Hobbs could kill Ivy that doesn't necessarily mean they were some sort of warning.  Could be that Mab wanted the fight to continue until Ivy was forced to flee to the NN (where Mab or something else might have been waiting).  It is also possible that the fight was designed to wear down Ivy so that she would have less energy against the Denarians. 

Anyway, all I am saying is that I think that the Hobbs posed a real threat to Ivy and that things could have gone much differently if Harry and Michael hadn't showed up and done what they did.  Of course this is assuming that Mab sent the Hobbs.  IIRC she didn't actually answer that part of Harry's question at the end.

So in thinking about topics like Lea, Christos, and Nic, I think I might have put a few things together...  The main inspiration for my conclusions is how during the conversation between Nic and Harry before the Rumble at the Shedd, Nic/Andariel seemed genuinely concerned about Harry's account of Hellfire being used at Arctis Tor, and considers such a thing a "contamination" if it's a member of the "Order."

Contamination is the key word here.   I am thinking that most of the “Black Council” operatives that Harry has a chance to interact with aren’t actually card carrying “members” but rather are contaminated somehow into doing things along the lines of what they want.  Contaminated by what?  Some kind of Outsider influence.  Vittorio Malvora was flat out possessed by an outsider, but maybe the same is true for pretty much every operative we have come across…  Others that are somehow contaminated, and possibly by outsiders (to some degree) would be… Aurora, Lea, Vittorio, and some more that deserve more elaboration… 

Thorned Namshiel might have been contaminated, but I’m thinking that Tessa is actually the major vector there, and he was working on her orders.

Since you have been talking so much about the contaminated line, I had to look it up.  And then because the Shedd fight scene is so awesome I had to keep reading.  That is when I noticed something wierd.  After Tessa has captured Harry in the aquarium and is trying to get him to reveal where Ivy is, she says to him: "They say to give a man three chances to say no" (Small Favor Chapter 32). Now it may be just a coincidence but that immediately struck me as awefully similar to Cowl's: "Thrice will I ask and done" (Dead Beat Chapter when he is asking Harry for Das Lied Der Erlking.  I don't know if it has been mentioned before but if there is a connection between them, then it would certainly seem to support your theory.

I think you are on to something here but I think you are starting from the wrong place.  Maggie was involved in some sort of plan with bad types but I don't think she knew about the outsiders angle until later.  The start of this plan probably has to do with the meeting that Eb mentions at the end of Changes where Maggie invited him to a dinner to discuss a plan and then Lord Raith and Arianna were both there.  Eb specifically says that Maggie was pitching a plan to him which he didn't want anything to do with.  What was that plan?  Both White Court Vampires and Red Court Vampires can live without actually killing humans so they might seem like decent groups to try to make a deal with against more dangerous supernatural types.  Second, both White Court Vamps and Red Court Infected are kind of half mortal with a dangerous evil spirits in them that give them incredible strength.  Thus maybe she got sold on trying to breed some weird hybrid vampire/wizard super race that could fix the world.  Besides, if you think about it, we know of exactly one thing that came out Maggie working with Lord Raith which is Thomas.  If Thomas was in part the result of an experiment in wizards breeding with WC Vamps then it might explain why she was willing to leave him behind when she ran or why Lord Raith might have protected him so much that she couldn't grab him when she decided to leave.