1) I believe the trigger to opening the door was having enough AT, which the Rotor Ghog usually would provide. Make sure that Marion isn't under any negative status effects that reduce her AT. Make sure she isn't 'cold' or 'fatigued' for example. If you have any items or skills that inflict 'weapon boost', you could try inflicting that on her and see if that brings her AT stat high enough for the door.
It's also possible that the event takes a communal pool of your AT scores, and not just Marion's. In which case, put the highest AT weapon you have on every character, inflict 'weapon boost' all around, and even use Random's 'Legend Slayer' skill for good measure. If that doesn't get the door open, I don't know what would.
2) For step #45, you need to come up from below the room by walking through a fake wall. If you zoom all the way in on the map, you'll see a thin white line that shows you the path you need to take:
