Right, I mean, maybe it was an intelligent undead with no clear reason to stay in the room other than the adventure text said so.
Yeah, definitely true. Just giving the designers the benefit of the doubt and assuming they knew what they were doing when they said this guy stays put. But of course, in game the DM must apply their judgment....
But the magic circle thing... I'm just hearing "so we cast magic circle at the door, and it worked," and this is a big problem why? Maybe if anything it could be a bit too low of a level (maybe 4 would be better) but I mean the spell gives disadv and protection vs. charm/fear/possession... come on, man. It's not invincibility. It's fine. Sometimes I'm astounded at how very differently different people can look at the same game.