1. Nobody can open these doors, it's just another horrible dead ending in the game.
2.Can't honestly say.
3. There is a doorway that leads underground in the mercenery base, it should ne be too difficult to spot because it is guarded by a large amount of merceneries.
Edit: Oops. I had thought you meant the entrance into the sublevel, not the 'millitary base' door
inside it. Sorry about that.