I really feel for you. Well, my old ginger cat had hyperthyroidism and lasted until he was 21. He finally died in peace on the sofa. He was so thin he was like a sheet of stripy orange tissue paper, but he was a happy kitty, and that's what counts. He had kidney failure as well.
I know this is horribly worrying, but you have a couple of weeks at least before you need to make decisions.Before you make any choices, wait for the blood results.
If they're ok, could you get the teeth treated and see how you go? Cats don't really need a lot of teeth and they might be able to whip them out fairly easily.
Thing is, this is a place in life where there shouldn't be any rules. And it feels partic hard to have to be the one to make the decision oneself, to have to institute miserable bloody rules, when one is the one who loves the kitty the most. Sorry, that's really inarticulate. But if I had to use a rule myself (and I can't bear to think about that for Mr C, my fat tabby), I'd go for measuring the happiness v. illness ratio.
Once he's stabilised and the abscess is cured, what dominates the kitty's life? Slumbering in a sunbeam for the summer or curled up hiding? Howling for extra treats or ignoring the bowl? Only you can see him to make that judgement. Only you can hear the purr.
Have you read the piece in the Grauniad today about your very dilemma? I thought it was drippy but others may not. Might help