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.