Poor Sassie. It needs a big push from her to get over this, and from what you're saying, she doesn't sound like she's got much push left.
There are alternatives - I take it these involve surgery to work out what's going on behind the scenes? If she isn't feeding herself the anaesthetic will be risky and as you say, she's also elderly which increases her risk.
The decision the vet is asking for needs to be made soon as it sounds like she's weakening, and if you choose to try every weapon in their arsenal, they need to cracking on.
This is a terrible position to be in, Cazzer. Only you know her well enough to be able to make the call if she's going to benefit, or suffer, from any further treatment.
Thinking of you both today, and be certain that Purrs will be here for you whatever you decide