You brought up some valid points and some facts of which I was not aware. I agree that Antiquorum's actions appear very suspect if not outright unethical.
I do maintain two two things however. First that this was an instance with internationsl complications is obvious. Definitively ascertaining ownership would have been complicated and time consuming. It is not entirely unexpected that Antiquorum chose the expedient course. Expedient, as you suggest, seems to be the perfect word describing Antiquorum chosen course of action.
The second point I maintain is that the OP's first point of contact should have been the police. He shouldn't have started with contacting Antiquorum and trying to prove his ownership. If the law had been involved at an earlier point of time, Antiquorum would not have had an opportunity to slip out the back door so to speak.
My comment on the Insurance Company was if they were involved via a claim made when the watch was stolen. If it wasn't insured then obviously it's a moot point.