There's no evidence a third party or foul play was involved, all the evidence indicates she was driving the car.
She seems to have given a fair amount of care and attention to hiding herself, by selecting a residential neighbourhood where her car would blend in, and parallel parking well enough that no one thought anything was wrong with the car or tried to investigate.
To me, this behaviour isn't consistent with psychosis, in those situations the person is unlikely to be able to drive at all. It's not to say she wasn't in extreme emotional distress.
I also note she didn't call 911 or allow the police to find her. People who are experiencing a severe medical event, including confusing, racing thoughts, will usually be anxious to find help, and I'm sure her parents urged her to get some.
I believe her husband when he says she had no previous symptoms: she was a very successful, hard driving executive, apparently since age 18: that kind of person can't be showing overt signs of bipolar disorder.
She seems to have been incredibly ambitious and driven, to have done what she'd done by age 33. Whether that all caught up with her, or she had some kind of meltdown from more recent pressures, IDK. But my speculation is that her death was deliberate.