If you take over all 12, everybody can still make the 1 witness change. And when that is stabilized, the MainChain is moved some amount of set points (think this is a constant in the code), the next witness can be replaced.
I think it can't because it would make the witness list differ by 2 with regard to the remaining 11 witnesses. Otherwise there would be no way to form a cartel and no reason for schism. You would just change all witnesses one by one. But this can't be done as I mentioned and that's why schism is necessary.