Well, that's a bummer about Miss Kitty, but it sounds like the only real good option. Like you said, she would probably die if she tried to have them. Hopefully the kittens will make it, but I can understand how they wouldn't. Cat gestation periods aren't that long, so proportionally speaking when compared to humans, every day early is a long time as far as development goes. But you did the best for her, so that's what matters.
Yeah - the kittens would have killed her. She's just too tiny and thin to be a momma right now. They'll have to be bottle fed, if any make it. I'm not expecting that, but we'll see.