If TR was fasting for any length of time, it's possible she was experiencing hallucinations, both auditory and visually. It's quite possible a visual hallucination caused her to drive into the pond.
Many things beside psychological issues can cause hallucinations: drugs, fasting and isolation. If you combine all four she was ripe for experiencing them. I struggle with why her family members did not get her medical help. Her behavior for several weeks prior to her death would suggest a medical intervention.
I'm trying to put myself in her family's place. If my daughter, wife, mother was behaving in the same manner she was: walking the beach addressing complete strangers about God, acting in a manner that suggests she may have been having a psychotic episode, fasting to the extent she was endangering her own health as well as others, it would have set off alarm bells. She needed help, an intervention. I don't get the vibe that any of that happened.
Unless there is actual evidence that her vehicle was involved in an accident that sent her into the pond, I doubt the autopsy and toxicology results will provide any information.