I don't think your husband is trying to be abusive (unless he has a form for that, which you haven't mentioned) I think he is trying to hold his sanity together while he grieves for the daughter he once cherished who is now considering mutilating herself and cutting her family off.
But equally, his sanity isn't your responsibility. If more things need 'doing', then he can 'do'. He can't wake up in the morning with a list of things for you do to and if they work he can take credit for thinking them up and if they fail, he can blame you for not doing it or not doing it well enough. If he has an idea, you need to deflect it back and say 'yeah, why don't you try that'.
I can't say what I would do in your shoes, because none of us know how we would really react, but I like the idea of what some previous posters mentioned and just quietly make your home life enticing. "We're going to take the Eurotunnel to Paris for the weekend, let me know if you want to come, otherwise I'll send you some pictures xxx". There's no pressure, almost indifference, but letting her know the life she could be having. I imagine it's pretty crap where she is. It's awkward living with someone else's mum and teenage relationships are almost always turbulent. I would keep up with the drip, drip, drip of her nice former life "morning darling, today I'm meeting Glenda for a coffee, I haven't seen her for a while, so it'll be nice to catch up", "I hope you've had a nice day today, we played Monopoly after school and DS drew a picture of a tree, his art is really coming along [pic attached]", "Dad made a cake today but is so messy he ended up with icing sugar in his hair - I haven't told him yet! [pic attached]". No pressure, no oxygen to the trans situation, just normal family life, the same as you would if she was gone for the night on a sleepover or moved out to go to uni.
If your friend wants to call your daughter 'he', that's not a battle I would be fighting, but there's also no way I would be pretending she was a boy. Otherwise if she did change her mind, she'd have to eat her words and tell you to call her 'she' again, which is like admitting she was wrong. I wouldn't put that hurdle in front of her.
The therapy sounds good, and I agree to doing it without DS to begin with so you can check out the therapist first.
I would be putting any spare time you have (but not all your energy) into making things nice for DS, his life can't revolve around his sister. Ask him who he wants to invite over for the weekend, for example, or tell him you've found a new trampolining club that he might want to join. And I would also be getting a kitten if DS wants it. Or maybe he wants something else: a drum kit, guitar lessons, involvement in the Duke of Edenborough award, picnic in the park, anything to engage him and make him feel special. Just nothing computer related. Boys seem to withdraw into themselves when given free rein of a computer.
Oops, that was quite long!