Claim against whom?

It's not the fault of the original errant driver that repairs have taken so long and I doubt your contract with your insurer makes that provision.

I don't know why we've got three threads on this same topic: I'll close the other two.

The straight answer to the insurance question is that you need to check the exact terms of your policy. Policies differ and I'm afraid we can't help as we don't know what yours says.

I personally think that as your car is five years old, being offered a second-hand seat is not unreasonable particularly given that a new one seems to be unavailable. You should of course have the chance to look at it yourself and make sure that it's undamaged / not excessively worn.

If you are about to sell the car anyway, all you need to do is make sure that the seat looks and feels the same as its friend on the other side of the car.