I've played the "what if" game way too many times, and it isn't a game you can win. In the last year, I've lost someone very special and several pets. Some of the pets got vet care -- lots of it; some just died suddenly.
I have to remind myself that not only is it unlikely that I could have prevented most or all of the unhappy outcomes, it is absolutely true that there is nothing I can do now. Regret can eat you alive, but it can't change what's happened.
Take care of your remaining chicks, and please, take care of yourself.