Very interesting post and backed up with pictures to explain which is great.
OK firstly have you written to Renault customer service or is this just an issue with a poor dealer unwilling to assist you ?
From your pictures it would appear that "maybe" the door paint has been damage by the seatbelt ?
It may be worth getting the written opinion of an independant bodyshop or paint specialist to back up your warranty claim.
Manufacturers have to unfortunately guard themselves against warrenty claims and therefore tend to argue the toss rather than hand out the dosh. You sound like your not going to be easily worn down by them and good on you. Maybe someone passing by this forum who is experienced in bodywork can offer some advice ?