Wow...you and your cat have been through a lot.
I recently lost my first cat after 20 years of being friends and she had similar problems that you are describing.
A few things to note:
1. once we get her eating again, speak with your vet about y/d. This is a new food by Hill's that treats hyperthyroidism. I wont get into how it does that but trust me, it is amazing....no more pills or transdermal gels.
2. The chronic vomiting and anorexia could be due to a low grade but long term pancreatitis. This inflammatory condition is very tricky to diagnose in cats however a special test from Idexx (the name of the laboratory, they service all of north america) is available and it may shed some light on the subject.
Lastly, I am a little worried about cancer in this case. Your vet has done an amazing job assessing organ function and performing diagnostics like the ultrasound you mentioned, but with a cat of this many years and with normal kidney and liver enzymes, cancer is a real possibility. My cat coped with generalized abdominal lymphoma (GAL) for 2 1/2 years and even a board certified imaging specialist was unable to detect it at first by ultrasound. GAL results in the smallest little tumors within the abdominal cavity and over time cause gastrointestinal upset, sometimes diarrhea, and definitely weight loss. Treatment involved supportive management of the symptoms (cerenia is a great anti-vomiting medication you could try), dietary management to help with weight gain (she was eating a high calorie vet prescription diet), and initially prednisone and metronidazole to reduce the inflammation of the abdominal tumors.
Ask your vet if this condition is a possibility.
Good luck, I really feel for you and hope everything turns out better.
Dr. Cliff Redford