I believe the serial number of my Mavic will be on the box (which I still have).
I have registered the Mavic with my email/login credentials.
I also have the controller (and iPad)
1) - Can DJI tell / (and possibly inform me) if someone else ever tries to register my drone?
2) - Can DJI prevent any attempt to re-register the drone?
3) - Can the Mavic be flown without registering?
Sorry to hear your Mavic was stolen, please report the issue to police to see whether they can help you handle it.
Since you have activated the drone, then there is no need to re-activate it if someone else had it.