I am so sorry to read about your cat and I understand how you must feel.You must be devastated.
I have no medical training whatso ever , so am unable to comment on the treatments your cat has had. But from my own experience of having animals , I think that a vet starts from the simplest , most basic condition as a possibility, and then moves onwards on a scale of more serious possiblities , prescribing whatever may be appropriate to the conditions at the time.As a treatment fails to bring an improvement in the animal , a vet must then look for really serious conditions and prescribe accordingly.
How could a biopsy not reveal the facts , quite easily I would think , because they must depend on the state of the underlying tissue at the time of taking .A tissue analysis may reveal something on one occassion and something more serious later.as things develop.
I can understand that you feel very angry that an accurate diagnosis was not made first off, but perhaps this was not clear cut in the initial stages.
Has your pet suffered in the interim , I don't know but would think not due to the medications prescribed, which could have contained an element of pain relief.
Suggest that you discuss how you feel with the practice ,and they should be able to explain their methodology of treatments, which may help you in this difficult time.
Sorry to be so long winded in his post but was trying to help.
Sincerely Brenda.