They replace with US spec parts:
- headlights, and rewire corner lights to be park lights as well
- tail lights, the US version has a red lens on the side
- bumpers, heavier must be safer
- km/h speedo to mph speedo
- third brake light added
- side intrusion beams in the doors added (to make it difficult to work on the windows)
The rest would be emissions stuff.
You might contest the PO's claim on the basis that he/she is not qualified to judge the safety of a car.
As a last resort, maybe you can request a refund on your premium payments since they were probably based on a payout for a US spec car. Worth a shot.
If everything is not absolutely done right and the paper trail isn't complete, it can be a hassle to get the car inspected.
I don't know if dealers still won't touch a gray market car. I don't think independents care.
All the MB buyer guides advice against gray market cars except for collectible cars (e.g., 450SLC 5.0)