Now you've got me wondering about how evilBillionare would do a fediverse takeover.
As you point out, buying an instance or 10 wouldn't do it.
So, I think that leaves propaganda disinfo pushes (which we already see in the fake account spambots), building a recommendation algo that well-meaning people end up using and can be twisted, or building a big referer network / link sharing system, ala Flipboard or surf, and getting people to use that instead of the native Fedi tools.
Hmmmm ....