Can you ask for Buprenex for the mouth pain, whether it is a tooth or a sore, it could help. If that is the problem with the appetite, it will help immediately, if it doesn't then the problem is something else.

Also, can you get her on A/D + nutrical or Feline Rebound liquid Diet?
These have added calories and can be syringed.

Hoping for positive outcome for Miss Sasha.

Be strong for her.