From the RRG p11: Doors

* The spaces that share an edge with a door are the only spaces that are considered to be adjacent to that door.

Does that mean that only the two spaces on each side of the door is adjacent to it (ie. 4 total) and not the corner spaces (like in Descent: ie possibly 8 spaces)??

On some maps, that make quite a difference

Correct.

Correct. A LOT of people miss this and it makes things much easier for the Campaign Rebels if you allow them corner access. Suddenly, plugging up and defending those doors is way easier.