I would imagine there are cheaper ways to accomplish the same goal

That still does not answer the question.

How would you find a definitive answer?

That is a very good question, monitor the SEC for clues, if they bring some kind of action (which should either happen soon now that he has made it official he wants to back out or not at all).

given the SEC's regular lack of enforcement, their inaction would not be evidence of lack of wrongdoing

With counterparties like Musk it doesn't pay to try to go for some minor infraction, they will just lawyer their way out of it or stretch it forever, but if it is solid enough (and $44B might just do it) then it may well wake up the dragon. To be fair, someone would likely have to ask them nicely to do so because they feel that they have lost a lot of money due to Musk's actions.

Which ones? Basically any way of him dumping billions of Tesla stock would've immediately popped the price. With this action, not only did he get plausible deniability, he also got a massive distraction on top.

Create Mars Foundation or do any other shenanigans rich people normally do.