From "Descent rules":

Important: Monsters cannot open any door leading to an unrevealed area

From "Descent-v-1-3":

Named monsters may open and close runelocked doors, whether or not the heroes have the runekey to that door.
However, named monsters still cannot open any door leading into an unrevealed area.

Question:

Can monsters in an unrevealed area open a door to a revealed area and if "no" what is the point with the errata?

Monsters in unrevealed areas do not exist and thus cannot open doors.

Palather said:

Because the heroes could close the rulelocked door thus trapping any unnamed monsters chasing them on the other side. The clarification makes sure that you know that named monsters (usually the bosses) can open those doors again it they can/want. (one example)

And, as correctly stated above, monsters in unrevealed areas do not exist.

Can unnamed monsters never open rune doors, even after it's been unlocked? (i.e. after the heroes find the appropriate rune)

I thought this rule was in place for the circular type quests, where if the heroes go down path B, but there are named monsters behind path A and a runelocked door separates them (the heroes having revealed the area but decided to go the other way and not go get the rune). Not sure if there are any quests like that in Descent, but Doom has one, the very first one. The blue door in area 4 the marines could break the key and not be able to use that door so have to go the far way around. If the Invader can open that door (which he usually does with a Smash card) he can spawn while the door is closed once the marines get to the final area, smash the door then move his new creatures in. Does something like that exist in the current Descent Quests?

-shnar

Unnamed monsters in Descent can never open or close runelocked doors, even if they've been opened or closed previously.

The errata allowing named monsters to do so mainly prevents the heroes from opening a door, launching a bunch of attacks into the room, and then closing the door so the boss can't retaliate. The "even if the heroes don't have the rune key" part can matter if there are multiple routes into an area, as Shnar said.