1. Caster must be burning OR on necrofire. Otherwise skill cannot be cast (should be deactivated like jump skills when my speed is 0)
I suggest you have a look at the stats for Combustion as it is a skill that does additional damage if the target is burning. It might not exactly meet your needs, but it might be a good starting point.
Quoted part is actually about "making skill not usable at all" if I don't have certain status. For example making it impossible to cast InfernalFires if I am not BURNING.
As for your suggestion to look at Combustion - I have no problems big problems of adding extra damage if my TARGET has some status. Biggest problem is in adding extra damage to my TARGET if SELF has some status.
Imagine a Firelord that casts InfusedFireball. Normally InfusedFireball is nothing but a normal Fireball but if Firelord is BURNING then all targets of InfusedFireball will get 50% extra piercing damage and NECROFIRE.
This is the one that is hard to implement