Here are the directions to fold the 3rd row seat into the floor:
1: remove headrests
2: push button on back of seat while pulling back on the pull strap. This should rotate the seat back towrd you.
3: pull the handle under the seat. This should “collapse” the seat.
If it doesn’t then you may have something blocking the seat or the handle may ghave been disconnected. I’m not sure about the cost but I would check with a body shop first before going to a Nissan dealership, mainly due to cost. The hourly rate at a body shop is much cheaper. The body shop may say they can’t do it.