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.
A cartel / collusion, I think is if some of the 12 witnesses would cooperate behind the scenes. That is ground for schism. Because then it would take too long time to change them. If 6 witnesses are in collusion and are actually acting as 1, they can deny any forward movement at all on the network - breaking it.
edit: THIS BELOW IS INCORRECT.
All 12 can be replaced, it just takes longer time, exactly to prevent changing all 12 at once to "Great Enterprise Censored Network". This is the point schisms addresses in the whitepaper.
Just a moment Ill find whitepaper reference to how more than 1 witness can be changed - there is a rule, when 1 witness changed becomes stable enough for the others to be allowed to change. EDIT: 1 at a time can be changed when no majority of witnesses are evil.