Couldn't he assign the janitor to be the official 'CEO', but then give himself title of 'Visionary' and still make all the decisions?

The SEC would not find this amusing.

Part of being a visionary CEO is making choices that other people don't like, and sticking with them. As a janitor, you can suggest all you want, but unless you're on the Board, you can't force things through (unless you rely on backchannel bribery or coercion or other unsavory means).

Musk is also on the board.

But the SEC is also seeking to remove him from the board of any public company. So he'd lose his chair and the CEO position in this single case.