I'm so sorry, Carson. It's always difficult to lose a pet, and even worse when you have no idea what it could have been caused by. Such was the case with my sugars; when I lost them, I had no clue what was going on, and it happened just so fast. I was an assistant at my little boy Bixa's necropsy, and saw some of the damage firsthand. It was so frustrating to see what the sickness had done to his body when I had no clue what caused it (or what I could have done), but thankfully the exam helped lead us to the final toxicity conclusion. Hopefully your vet can find some information that can give you a better idea of what might have happened.
You did all you could have done to help her; most people wouldn't go so far as to have the nec. done and find out what happened. Best wishes and condolences to you and your family on your loss.