Joe wasn't stupid, but he was old, sick, and not nearly as sharp as he was at one time. He was very obviously in decline.
And insofar as the state putting words into MM's mouth, we all have heard the speculation of how they may well have had incriminating evidence against him due to his personal life. We all can see how his story changed over the years. We have all seen how crooked the whole system is. It is very easy to connect the dots and surmise that when MM's story changed, it wasn't him alone who changed it.
But of course, you will refute that.