I found his memory loss totally plausible. Real age related disability makes sense in the case of Mueller if you think about it.
First, he was compromised in the past with 9/11, U1, god knows what else, so he would be a prime candidate for recruitment even if he were not senile. Second, his reputation among the chattering class was such that his name would lend credibility to an otherwise obvious charade. Third, when the scam is revealed, the cabal can point to the demented old man at the head of the investigation and blame him for the farce. Fourth, the senile guy is cover for the real work being done by the Clinton lawyers and investigators in the background.