Kennedy was driving. This is a theory that it was the only way he could put it behind him was to say he was driving if she was, etc.
Joe Gargan was Ted Kennedy’s boyhood friend and “fixer” for most of his adult life, the one who was at the bridge later with Kennedy before he jumped in the water and swam over to Edgartown.
During the heated discussion on the shore, where Kennedy was lying on his back in the sand, trying to figure out how to save his political skin, he jumped up and exclaimed “Why can’t we say she was driving” and was immediately squashed on that.
Joe Gargan was later ostracized and thrown under the bus by the Kennedy clan for speaking the truth on the events. (he only knew what happened from the point Kennedy appeared back at the cottage saying there had been an accident)
https://www.youtube.com/watch?v=SEOd7AOR6ic
Here is the documentary that I mentioned in an earlier post (Chappaquiddick - Ted Kennedy Documentary narrated by Ian Holm). It was made in 1994. I think the theory posited in this documentary answers most of the questions that other theories don’t with the exception of one.... how did he get back to his hotel. However, there are probably quite a few logical explanations for that detail.