who is/are corrupt ...

I think "who" by itself is generally singular, though it can be plural

It shoud be "are"

People are watching and we know who is corrupt," he said. "If Najib as prime minister choose them as ministers, his political party will lose the next general elections."

I could go either way on is/are. I can see why are would be correct, but I would probably say 'is' myself, as if I were looking at each potential minister individually on a case by case basis, rather than at all of them as a group.

I also personally feel that sometimes going with what I've been taught is correct, but doesn't come instinctively to me, sounds stilted and 'put on'- although I always try to remember all those rules when I speak/write to a friend of mine who was my English professor.

I think that's a good point for people who are learning English as a second or 'other' language to remember too - sometimes sticking too closely to the lesser known (and used) rules of grammar sort of 'brands' them as a non-native speaker.