Oh bless her.
Poor Pebbles. <3
I honestly don't think it would be worth taking her for her teeth to be done. As you said, as fragile as she is, I don't think she would cope under anesthetic. I would just continue as you are, keeping her content and comfortable.
There's no definitive way of saying how long she might have, all you can do is keep nursing her as you have been, so she is comfy and happy. You're doing everything that you can.
Sending love to Pebbles and you, give her a snuggle from me xxx