Laura, I'm so sorry for your loss.
I think almost everyone who goes through something like this feels guilt and second-guesses the choices they made. The "right" choices feel so obvious in retrospect, but there's really no way to know if it would have turned out differently. This honestly doesn't sound like an issue that the vet knowing his history would have mattered. Either vet should have been able to diagnose and deal with this.
You did your best with the information you had at each step. Try to have compassion for yourself and focus on loving and supporting your sons. Again, I'm so sorry.