I am so sorry to hear about Sassy and its one of the hardest things
To be honest I think you should write to your vets and also ask for copies of her notes, I did this for my Kocka and found the vets very wanting. She had a tumour on thyroid about 4 years before and it was removed, she never put back the weight and getting her to eat was a constant problem.
I was very unhappy with the vets care here and she went to the Bridge only 6 months after moving and never got the care that she had in London.
I would also ask for a copy of her blood test results, which they should have given you anyway.
I suspect that they did not fully diagnose everything that was wrong with Sassy, which may have been hard to do but I believe its unusual for a cat to go downhill so quickly after just being diagnosed with HyperT.
My own feelings were that I wanted to know what care Kocka was given and be reassured that it was good and was not expecting for her care to be so lacking and for the vet to resist giving me the info that I had a right to receive and consequently the drawn out process made me feel much worse.
Grieving is a complicated process and I know that I proberly will grieve for her for ever and blame myself for not being able to get her better care.........I couldnt drive at time so had to rely on others
I hope that you will the best way for you through this, guess its a balance between knowing and not wanting to know and my thoughts are with you