For all practical purposes, doesn't Shotwell run SpaceX and Elon just shows up to give "input", whenever the capital raise du-jour is necessary, and to do the marketing and PR?

It does seem to me he will need another Shotwell for either Tesla or Twitter, though. You can be CEO of as many companies as you want, but if the CEO is also doing the Operations part of the work, then you're constrained to pretty much one company at a time. I wonder if we'll see Jack come back to Twitter.