She is 12 years old, so I am hoping she still has a few good years in her yet. I think the average age for an indoor cat is about 15 with some living to 20 or even more.
My cat lived to 18 years. Also, when he was 11 he had to have a lot of teeth removed, so also an anaesthetic--but he was fine. The vet had tested his blood before considering the operation and the anaesthetic, though.
I hope that MC is going to be fine and it's nothing serious. I am sure you worry. I have a three-legged cat too and I know how crazy I get when there's even a tiny problem with his health.