It sounds like a heartbreaking experience for you all. Having lost our older boy this summer quite unexpectedly, about all I can say is that bad things can happen very quickly and the guilt can feel overwhelming. You took her to the vet each time and did what they recommended. If she was that unstable the first time, they would have said she should stay rather than give you a choice of leaving her or taking her home, where you knew she would feel more secure. In other words, you pretty much did everything you could have done even though the outcome was not what you may have hoped for. Whatever the autopsy shows, it was not your fault and there was probably little else to be done. I am so sorry for your loss and my heart goes out to you. Rest in Peace, dear girl.