First I want to say I am so sorry that you have to go through this and I wish you the best of luck turning her around.

OK, now to wrack my brain for ideas.....

My first suggestion was to be nutrical, you are already there. GOOD!

The next thing is forcefeeding. Please DO ask about it because I have found that sometimes once a cat stops eating they don't ever start up on their own again. But once their tummy starts to feel satisfied again (via the force feeding) they WILL start to eat again.

The next step would be for your vet to insert a feeding tube. Extreme, yes, but at only 11 y.o. she should be able to spring back from this episode.

Other foods she might eat: Canned baby food, ckicken livers, dry dental diet food (for whatever reason, cats LOVE this stuff), meatloaf, kitten replacement milk, cooked chicken and rice.

Now for the behavior......if this started via a stress episode maybe we can make her feel more safe.
-Have you moved her food dish? If yes, maybe put it back, if no, maybe move it to a new location.
-Do you think if you confined her to 1 small room she might feel safer, more in control of her surroundings?

well, right now this is all I can think of. But if it were me, I would definitely go with the force feeding as the next step.

Keep us updated.