For example, Kemmlerian Necromancy is about as evil as you can get, and in narrative terms is really just a very powerful school of mortal magic. So yes, I'd say using that to break the laws would change your soul and get you Lawbreaker.
Hellfire and Soulfire aren't independent power on their own. They enhance and alter your existing magical talents. So you're still using your own magic, you're just getting a boost from the infernal or divine realm. As such, definitely Lawbreaker for using it to break the laws.
Yeah, you're already a Lawbreaker if you're picking up Kemmlerian Necromancy. Although, if Sponsored Magic doesn't Lawbreak across the board, it might be a good way to preserve some of your aspects while still utilizing necromancy (not that the Wardens would pay that distinction much heed).
If you've got hellfire, you're probably a monster already who doesn't care about invading minds; if you've got soulfire, you probably care quite a bit about not killing people.
I can see pure sponsored magic avoiding lawbreaker in some situations, but that the cost and limits of it being only at the whim of the sponcer should be a similar set of limits and drawbacks (I've had the discussion here before), but it also seems from the books that if you have your own magic, sponsored magic adds a boost on top, and you are still fully libel. The one or two points you saved by already having mortal magic are the points that would have carried the ignore lawbreaker with this specific power only effect... (the effect you get with the vampire feeding and mind control powers as already priced in)
And I can agree with that - if you're using it as a boost to your own magic (narratively), then you'd be subject to Lawbreaker stunts. But if you're a Knight of a Faerie Court - designated specifically to mess around with the humans the Ladies and Queens can't touch - I don't see why you couldn't fireball some guy with a gun.
I guess the discussion is a bit more fruitful! The general consensus seems to be:
1) You can't take Refinements.
2) You can take rotes.
3) Up for debate.