How to protect Relay's NC contact tip from wearing out?
Where you add the diode matters and it should normally be in parallel with the inductance that's producing the arc in the relay, not the NC-COM relay contact. The problem is since your circuit is designed to make a spark that can jump across an air gap so adding any snubber, either diode or RC, in the proper place to suppress that spark across your relay could also suppress same ignition spark. Even then, a 1N4007 is too slow. Silver cadmium oxide contacts help and are more resistant to erosion.
You might want to look at a thyristor which is unable to interrupt the spark while it is in progress (so none of the harshness invovled with interrupting something mid-arc), but will block any future sparks. This is because a thyristor actually latches on when triggered and can't actually interrupt current itself. However, if the current falls below a certain level (ideally zero) due to the external circuit while the trigger is removed then thyristor will turn off and block future current from flowing