I see so it’s been going on a long time.
I still think the best way to prove what has been done to your vehicle is to have an independent engineers report done. It will be much more detailed and helpful as evidence then just a mechanics opinion.
Also have you asked your own insurance if your vehicle has any marker for accident repair?
The only way it would show clean of a damaged repair marker would be if the car was repaired without an insurance claim.
So someone bought it damaged and repaired it themselves to use or sell on.
Another thought have you tried ringing dvla and asking for the previous owners address.