Hi all, I figure I should follow up on this because I got the problem resolved:
She tested positive for Bartonella (stage 3), which was probably the main reason of her teeth rotting, followed closely by her being feral for so long.
She was put on three weeks of antibiotics to resolve the Bartonella (as was my other cat, for prophylactic reasons), and had three teeth extracted and the rest cleaned. She is now happy as always, but also 100% healthy! Thanks for all your help.