The negatives or give resistance to negatives give people negatives or less negatives are taken. Example: If you have a 'resistance to negatives -30%' and the PC tells you the darkness modifier is -30% then you can use that to cancel out the darkness modifier.
These both could be on the same track except the most plus a PC can get from a spell is either +5% or +10%. The 30/60/60 is if you are building an artifact which is sentient and you want to give it as skill. Example: You are building an army of evil gnome statues and want to give them rifle skill - you have built up your skill enough to give them a 30% rifle skill.
Essentially, they're just broken down that way because I don't want NPC's to get a 5% or 20% skill - just easier to do it at 30/60/90. Though if we did combine them up (and just made certain things applicable only to certain things) that would probably be a good idea because then it is harder to get to 90%.
So the negatives thing is for every type of negative? You don't need to say "I have a resistance to negatives in darkness" or whatever?
It would be to negate a specific sort of negative, depending on what kind of spell you were making. For example, if you wanted a 'sure footing' spell it would be made to negate the negatives given by shitty footing. (I know because of rule 1 I rarely use negatives).
I wouldn't want to have it where it was a willx3 to resist the spell and then you slapped them with an additional -30% on top of that. Willx3 (for example) are hard enough to resist as is. So I'd say not for that though it is a clever idea.