just go to Nissan and get some touch up paint.
Wax is a paint protector. Best option if you have quite a few stone chips along with the scuff is a bit of a respray. Cheapest way is prob Chipsaway, You also get discount with them if you are a AA member
might not be the cradle but the base unit which is situated under the glove box behind the passenger seat. I would check the fuses for a first thing to test
not sure how the underside of the spoiler would look imo as its moulded for the boot lid where normally spoilers are designed to be more wing shape in there profile
check under the seats and make sure all the connectors are properly connected. Here can be an issue. If problem still occurs after a airbag reset procedure then you may have trapped the loom in the seat itself. I had this problem. Had the defective section repaired, cost £40