Wow, what a fascinating case. Funnily enough, "manic episode" popped into my head as soon as I read the first post. With the information from the more extensive article, it seems clear that something like that was taking place. Unfortunately, it would shock me if she did not die very shortly after disappearing. Episodes end, and, had she still been out there somewhere, it wouldn't have taken more than a few months for her to come back to her senses. The article even referenced that this had happened before; I think Margaret was aware of her mental state, to some degree, and took herself to Nantucket as she'd done in the past to curb her mania. She even contacted her brother and had him meet her, again, probably because somewhere in her mind she knew she wasn't well and she was trying to take precautions. I bet she probably even thought she could have kept driving when she met the Avon lady, but again, was making every effort to take advantage of moments of clarity and listen to what little voice of reason she had left. When she realized her behavior was upsetting people, again, just like she'd been doing, she made the right decision and took herself to bed to try to snap out of it. She must have been so, so deeply exhausted when she left the house that night; in her delirium, maybe a dip in the water seemed like it might cool her down and clear her head. I agree that she was not suicidal; on the contrary, I think Margaret, the intelligent woman she was, knew herself very, very well. She knew she was sick, in the way she was probably very familiar with by this stage, and did everything in her power to get better. Unfortunately, mania being what it is, she made one misguided mistake.
* ^ just my theory!