2. ... Does Cowel simply like Harry, or does he think Harry could be useful to him.
Cowl's treatment of Harry is a recurring topic online.
My own favorite theory is that Cowl is Harry (and Kumori is Molly), come back from the future. We're pretty sure there WILL BE a time-travel book, eventually, so we're expecting to see some Future-Harry action revisiting (one or more of) the existing casefiles; my theory is we'll see Harry-as-Cowl-POV re-takes on all the stories where Cowl appears (all the way back to Bianca's ball...) .
Cowl's magic is remarkably akin to Harry's, and Kumori & Molly share a "Japanese-esque" casting-cant. I think we'll see Harry cosplaying a Darth-Wannabe villain (to his mingled delight & chagrin)!
... 5. Peabody was not working for the same team as Cowel during Dead Beat, but some time between then and Turn Coat changed teams.
Cowl wasn't actually working with the Kemmlerites. He was one of their peers, and they seemed to think of him as "one of their own" (so presumably he had been Kemmler's student at some point). Onscreen, though, Cowl (uniquely of the necromancers) expresses only disdain and dislike for Kemmler, no reverence or respect.
Because of their mutual rivalry, nobody noticed that Cowl was actually trying to undermine
all the Kemmlerites, collectively; not merely each of them, individually...
Most likely, Cowl was already working with the Black Council at this point. That puts Peabody into Cowl's camp, I think, but not an ally or available to any of the other necromancers.
... 6. Kumori's offer was bs and they would've just take the book from Harry as soon as he set it down to destroy it.
Harry doesn't need to set it down, he could just Fuego it straight out from his hand, or boost a VentasServitas with Hellfire, or... Well, lots of stuff. And Cowl&Kumori know it. The book is a fraction of a second from destroyed.
What they
DON'T know is that Harry
needs the book (to pay off Mavra)... Unless of course Cowl is Harry, in which case future-Harry is just playing through the charade, because he needs "everybody" to know that Cowl is a badass villain...
... [Peabody] ... wrote [Die Lied der Erlking], so I think it would be kind of strange if he wasn't involved to some degree ...
I think the book is decades-old, as of Dead Beat. I don't think he wrote it with the Kemmlerites in mind, nor intending necromancy. It's just turns out that summoning the Erlking on Hallowe'en is their best shot a raising a huge number of potent ghosts.
They might have tried to take Peabody himself, but... well... extracting someone locked inside WC headquarters at Edinburgh isn't exactly their best shot...