Hi and welcome to the forum,
First off I would have to say that, in my opinion, you are being unreasonably hard on yourself. If your cat was elderly maybe she was going to have the stroke anyway, nobody knows.
You did a very good thing by taking in those two cats. Very few people would have done that - give yourself credit - that was a huge thing you did. I think it really stinks that someone would tell a cat lover (like yourself) that they were going to euthanize their cats "if nobody wanted them". What kind of crap is that? Why didn't they want them? That sounds like manipulation to me. They knew you would probably take them in.
It sounds like you still have all three cats, don't you? If so, I don't think it would be a bad thing to give up one or both of the new-comers to a no-kill shelter so you and your kitty can return to peace. You said you live in a "shoebox apartment" so having 3 or 4 cats sucks for the cats and you. I don't think giving them up would be unreasonable at all.
Just so you don't feel alone I will tell you that I am feeling really stupid and guilty about the death of my cat. I let some vet. give her a rabies vaccine when she was an indoor only cat, was stressed out, and was recovering from a severe food allergy. We had to put her down 3 weeks later when she had full-blown lympho-sarcoma. After a while I will be writing the vet. and discussing it here - not yet though - I just wanted to let you know that what happened to you could have happened to anyone.
This forum is a place to talk about very emotional, difficult situations, as well as happy, fun situations. I am sure you will have some of those happy, fun situations to discuss soon. Keep us posted on how things are going for you. Take care, Chuck