1) Geir will indeed be able to smash the marble, but he may have a difficult time actually getting into the building without destroying the palace to get to the bottom of the building. It has the potential to bring the Palace down with them if he essentially tears down the hallways and floors to get to the bottom, because none of them are suited for a dragon. That's a risk the Circle can run, but it may result in the death of the person you are trying to save.

(2) Yes! You can use magic on the guards and throughout the entire palace, but it may not be successful against the room itself, as the room is designed to ward off magic.

(3) You can certainly try!