1/2. I don't think Cowl and Mavra were working hand-in-hand. I think that both of them showing up in both Grave Peril and Dead Beat was mostly a consequence of necromancy being involved. Though I've often wondered if Bony Tony "acquired" the shipping container with the Word of Kemmler around this time.
That said, I do think you're right in that Mavra was understated in GP. I don't really understand what she had to gain by mentoring Bianca, unless she was attempting to manipulate the Red Court and the White Council into war somehow, but that's... not really a high percentage plan, unless she knew the future.
On the other hand, wasn't Mavra the one screwing with the Barrier? Because that's what let Kravos become the Nightmare (that and a dark ritual, which I suspect he put together with Mavra or Bianca's help), and also what let the ghost of Bianca's "friend"—the one with the inconsistent name—rise up to haunt her, which is part of what drove Bianca nuts. So I'd say it's possible that she was manipulating Bianca and Kravos to get to Harry and Michael. Mavra doesn't strike me as the kind of villain who fights in the open; she's seen too many Blampires get taken down by angry villagers for that.
Also, there is a lot of speculation about the origin of Black Court Vampires and the Outsiders, driven in part by this Word of Jim:
And also what is Drakul a scion of?
Drakul wasn’t a scion of anything! He was something entirely unhuman that got trapped in human form. Dracula was his half-human child, who naturally had enormous paternal issues, and wound up creating himself as the first Black Court Vampire in an effort to win his father’s approval.
It didn’t work out so well.
Some have taken that as a possible hint that Drakul himself was an Outsider who got stuck on the Inside. There's also the strange comment Harry makes to Lash about Outsiders and the Black Court in White Night. I just spent like half an hour looking through the Word of Jim archive to try to find it, but I specifically remember Jim responding to a fan's question about that, and clarifying that it wasn't an error (meaning that he didn't mean "Black Council," which some thought might have been the case, since Cowl was present at the time, and they were discussing Outsiders and Starborn). Regardless, it's a curious thing to link the Black Court to Cowl at all in White Night; there weren't any Black Court Vampires hanging around, so why would Harry bring it up?
But then there's Cowl, who gave Lea the Nemesis-tainted Athame in Grave Peril, which is linked to the Outsiders, along with Vittorio (one of Cowl's flunkies) being outright possessed by an Outsider somehow. So... yeah. I think there's a connection, I just don't know if they're working together.
3. Mostly, I think Bianca was crazy by Grave Peril (being haunted by a ghost for a few years could do that) and wanted to make him suffer. She also was looking for a legal reason to kill Harry while he was there at the party, and the best way to do that was to goad him into attacking—which he did. Ortega threatening Harry with war was, in my opinion, a representative of the Red Court trying to be the Adult in the Room, because they weren't ready for it yet—he's also the guy who tries to put the brakes on the war in Death Masks. I don't think Bianca particularly cared about the consequences.
