The MitD outwitting Xykon.

[Porthos]

but I'd still have a problem with certain characters going directly from hack-slash-kill to flowers-and-chocolates-for-all in the space of one strip, just because the protagonists find it handy.

But it's NOT one strip, as we have pointed out.

Consider, the MitD took the initiative to seek out Tsukiko to try to find out what happened to O-Chul. He realized that Tsukiko had the skill set to help him with a problem and fashioned a plan of attack to execute it.

This isn't the actions of a brain dead drooling idiot.

That he overlooked the Cloister problem just means he didn't take in the whole picture.

The MitD has been slowly coming out of his shell and acting on his own as opposed to being happy with being fed and lounging around.

His problem wasn't intelligence. It was that he was a underachieving slacker. Who may have had a huge blind spot in regards to memory on things he didn't particularly care that much about.