BenMax - you asked for any/all ideas....here is one thing I would definitely do:
(First, understand that I'm NO FAN of "prescription diets"....that said, there is one of the Hill's products that has a proven track record of helping severely nutrient-deprived kitties...)
Considering that she is taking in so very little - despite your best efforts - if I were in your position, I'd be maximizing the protein and fat content of the little that I was able to get into her.
The
Hill's a/d(link) formula is very high in protein and fat, is easily given by syringe and is considered quite "tasty" by most cats. I would, at least, pick up one can from the Vet and try it out.
(On a
dry matter basis, Hill's a/d contains
44% Protein and 30% Fat)
I'm rooting for this little one too!