I think the point in your last paragraph is an important one for you to think about. It comes down to intention. If CB went into 5A with the sole intention of abducting MM, then I think it’s fair to assume a planned process where a quick in and out makes sense.
I favour a situation where CB entered the apartment to be a weird pervert because it’s an unequivocal fact that that is what he was ... he had no prior convictions for abducting children and has had none since. This obviously doesn’t guarantee that he didn’t abduct MM or IG but it’s aligned to his known, proven history which IMO makes the pervert intention more likely.
So far as the door is concerned, GM and KM have mentally reviewed it countless times. It’s a small, seemingly unremarkable fact but it is a fact. Their nighttime routine was to leave the door just slightly ajar. On GM’s check it was half open. Again, this could have been a mistake, perhaps when they left for dinner on the 3rd it was left more open than normal. In my mind though, if we are coming up with theories, it makes more sense to have one that explains all the facts not just some of them.