Landrover defender

Just because it has a separate body from the chassis, it still should fail on seat belt prescribed area, so a failure from me if the seat belts are mounted on the seats

There not atached to the seat but the corosion stil measures in 30cm

Dont think the 30cm rule applies to seats. Unless of course the seatbelt mountings are attached to them. Only a general "is the seat secure?" check.

Yes it was also near the brake flexi pipes

Going from memory (which can be a mistake!) the seat belt buckles are not attached to the seats but ARE fixed to the boxes the seats sit on so if the seat mounting areas are rotted then so are the belt mountings. It sounds like the other garage have a poor understanding of prescribed areas.