I am sorry things are looking so dire - one thing I would say is that purring isn't always a happy thing, they also do it to comfort themselves, so not really a good indication. It is a lot harder when it isn't as clear cut, it was similar with Mabel, but you have to consider their quality of life all the time, not just when they are sitting, as that is when neuro problems are bound to be better. Ask your vet the honest question on Wed, my vet answers that now, although she shied away at first, and then have a chat with Swampy, I always ask mine to let me know when they are ready. Good luck, and we are all here for you