I emailed you, Xan, about the Pepcid. If she's not eating, she doesn't need much stomach acid, and excess stomach acid can cause or contribute majorly to inappetance. So I'd go back to the Pepcid.
B-Complex also comes in a liquid injectable form, and I agree at this point that she probably needs all the B's.
I don't know exactly where you are, but I can't be more than 2 hours away at most. And I would be ready, more than willing, and able to come help, if only for a day and if only to give you some respite.
Oh, and a feeding tube is not as bad as it sounds. I had the same reaction when one was proposed for one of my cats years ago, who had stopped eating. He died because I didn't do it. I'm better educated about it now. Not to say that you should do this! Just that when we're evaluating our options, we need to try to not dwell on the ick factors too much, even though it's natural.
Hugs, scritches, and prayers from the heart for you and Happy.
ETA: Didn't meant to contradict you, Claudia. I'm sure there are things about feeding tube potentials that I don't know. Sounds like you do. Maybe you'll elaborate because it sounds like an important consideration.