Not what any of us want to hear, ever, so completely understand how rocked you st be feeling now byt this news.
However. She has, you agree, been brighter in herself, and showing interest in food and in toys. I personally feel it may be too soon to give up yet, especially as they haven't been too sure about the diagosis, let alone the prognosis. Were you able to ask the vet about Complivet? Can appreciate how upsetting it will all have been, but may be worth a go.
I think I'd be trying all sorts of stuff to get some meat back on her little bones, as you've been trying to do. Am trying to recall what worked with our Suki when she was seriously ill and wouldnt stomach food. I think we'd tried very finely minced chicken soaked in gravy when we eventually managed to get her to eat again. Also maybe some finely mashed up sardines in tomato sauce. Little morsels of cheese, if she enjoys that - little and often is the key if you can manage it.
Am so rooting for your little Trixie - I know how much she's grown into your lives already. Am hoping there may be some other suggestions for you to try as well.