I suspect this is more a matter of laying groundwork for his defense, although it must also be said that anyone who lies as much as he has over the years--and thinks he can get away with it forever--can't be quite "all there."
I think so too. IMO, he's taking refuge in a "crazy" act. He may not be "all there," as you suggest, but he's "there" enough to face the needle or firing squad, IMO. If I'm right, MH took many rational steps to not only hide his lies but to cover up the crime as well. That demonstrates that he was rational (if misguided) and that he knew the difference between right and wrong.