I don’t think it is unreasonable for an owner to be absent and not leave their keys with anyone. I do not believe it is common practice for people to leave keys when they go away, on the off chance there will be an emergency in their apartment building necessitating access to their property. So that is irrelevant to the question of who is responsible for repairs.
The door is common property – if the OC had to damage it to gain access the OC should repair the door. I had first hand experience of this many years ago, when there was a flood in my apartment, and the fire brigade had to break the door down. The OC paid for the door and the splintered doorframe to be fixed.