The Douglas loss never should've happened. Not only did Tyson not train at all for that fight, but Douglas wasn't up until 13 due to a bad count on the ref's part. So, as you said, he took quite a beating, completely out of shape, and, even though it wasn't official, stopped a Douglas that was in the best shape of his life. He has more heart than he gets credit for.
You also made an excellent point with him being the most over/underrated boxer at the same time. Honestly, the Iron Mike that used to go run at 4 in the morning "because the other guy ain't", was pretty fucking beast. If he kept that level of discipline and dedication throughout his career, I don't know if he would've lost to Holyfield or Lewis.
He was tough as hell, and he had plenty of heart. When his discipline slipped is when he started slipping. You can see it as far back as the Berbick fight. He destroyed Berbick, but he looked sloppy doing it and he clearly wasn't the same guy he started out as.
People forget he was a small heavyweight. You have to be sharp to even compete with some of those monsters, especially when you're always giving up height and reach...most of the time, weight too.