Two additional points that might be a problem for the theory:

(1) Mab tells Harry that she doesn't know who took Marcone:

This is problematic for the theory I think because it requires a trip to Demonreach.  There is no reason for the story to go to Demonreach, except for Ivy, who is naturally Harry's go-to Accords Arbitrator. 

However, the number of powers that COULD contain Ivy at all AND need Demonreach AND be an accords member has to be very limited, and probably pretty close to exclusive.

To illustrate, Summer wouldn't need Demonreach, nor would the Lords of Outer Night.  Mavra or a full power Lord Raith would have no hope of containing her.  Cowl on the other hand probably could do it using DR, but Harry can't summon Ivy to arbitrate with Cowl.

Counterpoint:  Mab might be referring to the ambiguity of whether the Denarians OR the BC-infected Denarians are calling the shots.  After all, she isn't stupid and a big giant hellfire pentagram is what Harry would call a clue. 

Counter-Counterpoint:  There isn't any reason for Mab not to tell Harry this, if it were her belief.

(2)  There still is no really solid explanation for why Mab declared Harry her Emmisary Before Marcone is taken.

It really doesn't seem like Mab can just declare someone an emissary for no good reason.  Just seems like there has to be some crisis in the fae courts that allows them to call in special agents.  But, Mab tells Harry that he has been chosen as Emissary for the purpose of recovering Marcone.

That means that she had to know of the plot before it was executed, hours in advance, which returns us to (1).  She somehow knows about the plot, but not who?

Counterpoint:  Perhaps it rather suggests that Titania was the first to act.  Perhaps the Gruff's initial hit on Harry was BEFORE the Gruffs are named Summer's Emissary.  So, Summer takes a shot at Harry and fails.  Meanwhile, Marcone get's snagged and Mab declares Harry Emissary.  Titania goes with the Gruffs since they are already out and about hunting Harry.

Maybe Titania knows what Mab is planning viz. Harry and is engaging in some pre-emptive strike to foul up Mab's plan. 

OR, perhaps it means that Titania is in on the Denrian-Demeter plot against Marcone.  Titania's part in this is to get Harry out of the way BEFORE Marcone is attacked.

Mab may have a bit of Intellicus about stuff affecting her. I think she knew something was afoot with the Denarians taking Marcone and possibly the Archive and it was be bad for her. She likely wasn't sure which faction of the Denarians were doing it at that time. Nick's faction (pro-Lucifer) or the BC faction. She would know Harry is the best choice to foul the plot/benefit Mab and announced him her emissary to force him to be her emissary since Summer would be out to kill him he might as well do the job.

I think it was a bribe and an appropriate gift.

A treacherous gift could give interesting results though. I still do not know what the treacherous gift to Lea did achieve for the Black Council but it turned Mab into an enemy eager to use Harry against them.

If they made Ferrofax into an enemy......

My guess is the gift to Ferrovax was either a bribe to have him leave the Black Council alone or a cursed gift that will screw him over and he might team up with Harry vs the Black Council in the last books.

I thought of that party as the first "open" move of the black council to spread chaos/gain allies.  It clearly impacted Lea.  Considering that the gifts were coming from the Black Council, I would assume that the recipients were not Black Council members.  But of course, this causes some confusion in my thought process about the Black Council.  Here are some of my thoughts:

Assumption 1:  The meeting/organization that Margaret wanted to start was the big bomb drop by Jim, and was the start of the Black Council.  Thus the start of the black council must have included Maggie, Lord Raith, and Arriana; and least originally started for "good" purposes.
Assumption 2:  Margaret left the black council and birthed Harry as a child of the stars for the purpose of opposing them
Assumption 3:  Cowl is Black Council. 
Assumption 4:  Justin was either Black Council, or a servant/pawn of the Black Council.  My guess is that he was an original member (since he was a peer of Maggie and if she helped start it he would be an obvious person for her to bring in) but may have been sidelined as the powerlevel of the members grew.
Assumption 5:  The council now, but maybe not originally, uses and may be corrupted/led by outsiders.  A guess is that it may have been the use of outsiders that drove Maggie out of the group.
Assumption 6:  The council was giving significant support to the red court (not just as pawns).  They did some serious outsider summoning and service with the Red Court and almost succeeded in defeating the white council 2-3 times.  Any of those could have succeeded, so I don't consider them patsies.

Problems/Questions
A.  If Maggie new all about the black council and wanted them stopped, why didn't she share their plans with people that she could trust?  Dad seems like a likely candidate.  We know the gem Harry recently acquired carries her knowledge of the ways, I wonder if she may have put some other knowledge in it as well, and he just has to ask the right questions...

B.  If Maggie was a founding member and was recruiting, wouldn't Lea be a useful member?  We know she and Maggie were tight.  If she wasn't a member, wouldn't Maggie have warned her (to help in the protection of her son) enough that Lea would have been smart enough to avoid the situation at Beanca's party (knowing the powers they deal with and that they are up to no good?)  I'd kind of been thinking that Lea may be a member, but also made a deal to protect Harry, and thus this puts Lea in a very interesting situation.  After all, it may be that a member of the Cabal might be in the best position to protect Harry from the Cabal.  Or, because Maggie put her in the know, and she was protecting Harry, she was a being they needed to corrupt/destroy and thus that is why they gave her the poisonous gift.

C.  If Lord Wraith was a founding member, wouldn't Lara now know their secrets?  She's a lot smarter than Harry in terms of finding every drop of information and every secret that help her hold power.  I don't think there is any way after she had the mind lock on her dad that she wouldn't milk every secret out of him.  And after what happened in the deeps, she has to know that the black council is after her, so if she knows, she should have shared information about them with Harry by now.  So she hasn't signed on with them (or the Malvora plot would never have happened) but she doesn't seem to be working against them- indicating that she doesn't know, which indicates her dad didn't, but that makes no sense.

D.  Arianna's involvement vs. the Red King.  In changes the king said about her, "Determined to cling to the past, rather than exploring new opportunities..."  That does not sound like the desciption of a black council member to me, that sounds more like a black council member (or want-to-be) talking.  It makes me doubt who it was the black council was manipulating - arianna or the king?

E.  The stories, especially the short stories, have a secondary theme of the older gods/forgotten powers wanting to get back into the limelight.  I think that is starting to happen with the move of the Fomor.  I think it is possible that that is partly the changing of the order that the black council may have been originally part of accomplishing.  Sort of like a Society of Creative Anacronisms taking magical crack.  Maggie may have thought that with more of the older powers up and running and dealing with mortals that they would have helped break the stranglehold of the white council and help with encouraging reforms.  After all, a lot of the older powers aren't necessarily useful, and they may have believed they could use them.  It was only after they took that logic one step too far and started tapping outsiders that Maggie decided to get out.  After all, helping to restore the greek pantheon isn't against the laws of magic (And thus isn't mind-twisting), but dealing with outsiders is.

F.  Since the black council probably included powerful white court and red court members, it obviously wasn't opposed to vampires and could have included the black court (ie Mavra), but of course, considering that the White Court was behind the fall of the black court, it is unlikely that they would be open to including it in their plans to grab power.  Makes an interesting conundrum.  It is possible that the black court was specifically excluded, and thus might make the perfect enemy-that-is-an-ally.  And since they hate and are watching the white court, they could be clued into much of what they are doing already.  I have no idea what the party indicates about this: since Mavra was receiving gifts it seems to indicate she wasn't black council; but her teaching Bianca seems to indicate she was.  My really long-shot idea was that Mavra was working to oppose the Black Council by encouraging Bianca to go after Harry (and start the war with the wizards before the Black Council was ready) and the Black Council was trying to bribe her into allying with, rather than opposing the Black Council - either that or it entirely misread her intentions.  I think Mavra is very very subtle, and might be a lot more powerful than we know.  Her defeats were strategic and allowed.  If anything, they have lulled Harry into thinking she is a lot less of a threat than she is, and she is happy that way.

G.  Outsiders are corrupting the balck council.  The whole big shabang will eventually be about Outsiders, but it will take awhile further to get there.

I don't think the Black Council with Ariana or Lord Raith were ever good. Perhaps they were deceiving Maggie and others. I have no idea when the BC was founded but I think Maggie was invited in. I think Ferrovax got a very powerful cursed gift. Lea got the cursed Athame of Morgan LeFay. Ferrovax may want some vengeance on those who did that. Remember Lea talking about her being grateful to Harry for taking out Bianca, Ariana and the whole RC. Ferrovax may be in a similar grateful mood towards Harry if his gift was cursed. As to Lara knowing what Papa Raith knows about the Black Council perhaps she does or doesn't. It depends on how much was left in Papa Raith's head after Lara did the mind whammy on him. The BC did seem to want to take Lara out several times  with the Uberghouls and with Madeline etc. I think she is against the BC and is letting Harry to oppose them for her and her giving Harry low key support.

I can see Harry's mom as an early plotter into the BC. A BC that might have been more altruistic in the beginning and was quickly corrupted by more sinister members. That could suggest that a controlled use of outsiders was a possible plan of action.

Perhaps the BC started out more altruistic or not. I think they recruit useful idiots and idealists besides the evil black hat types. Kumori seemed the misguided idealist who is being a useful idiot. Maggie may have been recruited and by the time she figured out how bad they were it was too late and she was being hunted by everyone.

Wasn't the shout of fuego a guy ordering the mercenaries to fire?

You are correct.

Yes, obviously.  My point was different:  we can also infer that Harry's warding of the coin was effective at keeping Lasciel from influencing him directly while the coin was buried in his basement.  If Lasciel had been able to whisper in his ear whenever she wanted to during that period, I think it's not at all likely the Lash-personality would have been able to diverge at all.  Since Lash clearly became something other than merely Lasciel's proxy, I think we should draw the conclusion that Lasciel was not supervising during this time.

Assuming that Lasciel was the whispering shadow, clearly something changed about Lasciel's containment that permitted her to reach out of the coin and whisper to Harry.  We know that Harry dug up the coin and gave it to Father Forthill for safekeeping.  It seems pretty likely that the containment failure occurred after that--my question is when, how, and who was responsible for un-blocking Lasciel's ability to communicate with Harry.

If the answer turns out to be that Nicodemus walked into St. Mary's and walked out with Lasciel's coin, and has had it for some time, I think that would be a pretty big deal, don't you?

Who said the coin was at St. Mary's? Would you want a really evil, dangerous and powerful coin being where lots of innocent people go to church and pray or would you put it in a hidden stronghold? Nick and company has family retainers that are mortal and serve him. A group of them could have found out where the coin is hidden and gotten it or hired a groups of mercenaries or monsters to get it. It is also possible the church containment of the coins is not so good and a guard was corrupted. It could be as simple as they see who is a guard. Capture that guard's loved ones and offer a trade. We don't know.

  Uriel isn't evil, but he is an archangel, powerful beyond comprehension if he chose to use it.  Bob was merely warning Harry of that fact, you might say like Michael warning him about Mr Ferro..

I agree. As an example, was Gandolf in TLotR evil for getting Bilbo on the path to help the dwarves take out the Dragon and unite the Elves, Dwarves, Men vs the big Goblin army? Or Frodo going into Mordor to save the world? Would it have been better for Gandalf to leave them safe (for now) in the Shire then watch them die in the end when the world burns? Sure, Bilbo and Frodo were put at risk and danger but they were not forced to go and it helped countless lives and helped save the world. I view Uriel sort of doing the same thing with Harry. It has been mentioned in Ghost Story without Harry everyone on the planet is at risk, so Harry has to travel the path of danger to save the world and himself.