We also don’t know Thomas ultimate fate, he might be inprisoned, he might not, but I find it significant it is Harry’s memory of his father that is centre stage. There is a theory on Reddit that Eb killed Malcolm as Malcom couldn’t defend Harry and that Eb needed him out of the way to make Harry vanish into the Foster system, based on Journal and Ch 4 of PT. I give this credence, Margaret rebelled against Eb when he was training her to replace him as Blackstaff. I think Morgan promised her to protect Harry.....from Eb doing exactly the same to Harry. The realisation of what Eb wants for Maggie and Eb’s apparent willingness to kill (the doppleganger) Harry means that Harry consciously realises that Eb broke the first law to kill his father and put him through the foster system. He may have subconsciously realised this as far back as Dead Beat and the dream sequence with Malcom. The Jabberwocky was Eb. Harry’s subconscious is way faster on the uptake than Harry.
This is what has Harry depressed on Christmas Eve, we have foreshadowing in ‘will granpa come for Christmas’ in ch4 PT. Harry asked for the ring after his confrontation with Eb, and I think it was for this conflict which Harry which he foresaw the need. He told Eb he would kill him if he tried this with Maggie, Harry was deadly serious. I think on Christmas Eve Eb is in Demonreach, and Harry doesn’t feel at all guilty about that.
This makes Eb as a possible reason for Thomas attack on Etri more likely, to force Harry to see things his way. I don’t however think that Eb is Nemfected or Black Council, merely an abusive controlling asshole who can’t admit he is ever wrong, and has because of his power and status got away with it for centuries. He abused Harry and Margaret, in the name of his love for them.