There are fatal questions Mueller cannot afford to take the fifth on, and that is what he will have to do, because if he gave honest answers to such questions, his retirement would not be something to look forward to.
I’m not a lawyer, but it seems to me that if someone in Mueller’s position ever pleads the Fifth under questioning, then that would be an implicit admission that his entire investigation was a criminal enterprise. He would effectively be facing a default judgement in any civil lawsuit filed against him.