The ability to appoint a board member doesn't mean you get to control their every decision. Nor is their fiduciary duty just one shareholder.

Of course usually hand-picked boards don't go against their benefactor, and they're not keeping their seats.

That they did in this case says something about just how bad the situation must be.

Depends on the rules of the company. Just because he can elect the board doesn't mean he gets to direct their every decision. They are supposed to have their own independent judgement.

Depending on the company rules it is possible they might not be easily replaced during their term in which case they could do a coup in the short term.