You cannot buy enough Masternodes to have a realistic chance of subverting the network. There aren't enough DRK in existence.
And you can switch where your mining farm is pointed about 1000x times faster than you can set up a thousand Mastetnodes.
Yes you can.
A masternode serverfarm has to be setup too. Crazier yet, since you don't need to transfer the DRK to the masternodes, but can keep them external, imagine a really clever hacker who gets a few thousand unsecured webservers into his control, just to use them for this masternode takeover operation. He wouldn't even need to buy or rent the hardware...
He would just need to collude with rich enough DRK holders.