greene, I'm so sorry for your loss of Zoe.
A necroscopy (autopsy) might have given you some answers, but even with one, often the cause of death remains unknown. There might have been an underlying infection or a stroke or a number of other conditions.
It sounds like she passed pretty suddenly and didn't suffer much. Perhaps that will give you some solace once you aren't hurting so badly yourself. Time does help. Eventually the good memories will make you smile and push the sad memories further back in your mind.
The important thing is that you loved Zoe deeply and Zoe knew she was loved. That is a very wonderful gift to give any living being, greene. And she loved you right back.
You're in my thoughts and prayers, greene. We're here if you need to talk, cry, vent, or whatever it takes.
Zoe