He wasn't trying to short Twitter. He was trying to pump and dump. Though the dumping probably would have been done more tactfully than the standard pump and dump. Musk getting publicity for holding the stock helps to keep the value up. Tesla's been inflated forever, and he thought he could do the same with Twitter.

Yes, the primary reason he bought Twitter is that he screwed up during his pump and dump scheme. There are several reasons he was looking at Twitter in particular, including the jet tracking, how much he loves Twitter anyway, his hate for being told what he can and can not post, etc. I also suspect he's lost most of the people who will tell him the truth and is now surrounded by right-wing grifters (for politics and aiming his money, they're smart enough to not try to take the money directly). All these reasons were part of why the pump and dump was plausible as something else, and why it was Twitter that it happened to. Probably not even Musk knows how much of the intent to buy was legit (though not for 44b) and how much of it was the pump and dump.

He had a scheme; I don't know that I'd call it grand.