Oh My Manx!!!!! So sorry to hear about your Punkin. Fifteen years is a long time for Punkin to have been loved and cared for by you. She is one lucky little lady.
Anyway--my opinion only---Allow Punkin to live out her days being close to you and doing all the things she loves to do, even if it is just laying around on your lap.
We as caregivers must make the assessment as to what is best for our babies, and it is not always easy. I find that if a treatment (that may only prolong her life) is so stressful for them, then what kind of PROLONGED life are they going through all this stress for---Quality of life is most important...don't you think?
So in my humble opinion I would want to keep her pain free, home with me , being loved and spoiled until she is ready to leave her sick body and become your spirit companion and guardian. You have given Punkin a wonderful life and for that she will always walk beside you. Even in death
the spirit finds a way!!!
To you and your precious Punkin, enjoy the wonderful times together that are ahead of you, for how long as that might be.
God Bless you both and my prayers are with you during this very difficult time.
A Punkin Lover