Those are all great questions. What I think could narrow it down is this.
Where's the purse?
Did they find the purse in the house? If they did, that tells me this was deliberate. The reason I think that is because don't most women carry their remote start/key fob in their purse? If the answer is yes, then she had to have taken that out of her purse to get into/start her car, leaving the purse behind intentionally.
Perhaps the same would hold true if she was hit in the parking lot, the alarm went off, and she grabbed the fob from her purse and ran out of the house to shut the alarm off, however, why did those side airbags deploy? I doubt a light scrape along the side from another car would have set them off, but I may be wrong. I'm thinking the sudden impact from the car hitting the water from the side did that, but again, it's all a guess to me. If the car went in the river and was airborne for only a few feet, then the car rolled to the side, and then hit the water. that may explain it.
If they didn't find the purse in the house, or in the car. I'm leaning towards a robbery, or she left the scene willingly to disappear.
If they found the purse in the vehicle, once again, I believe this was an accident, and she's in the water.