But that's the thing: It's an attack spell and built as such, so if your players are doing it, they don't have to make it Aspect based at all. No spell to kill someone needs to be. A spell actually built just to inflict a single Aspect will never be that big.

So you think it's fair if the PCs spend a significant amount of time setting up the ritual--potentially spending their own resources and fate points--to curb an extremely significant advantage their opponent has on them, and the opponent then spends one fate point to undo all of that hard work, and gets to use all of his power at full capacity for the entire battle?

I have a problem seeing that as anything besides an utter waste of resources on the part of the PCs.

What's the narrative justification for the spell simply failing, despite the PCs spending all their time and effort on making sure it's cast correctly? How is it fair for the PCs to then get stomped because their preparations just went all right out the window?

I sure as hell do. Why wouldn't you? I mean, it's one to two at most for most enemies, so it's not like it's that hard...

Heck, even if doing the pool method (which seems a reasonable alternative), this'd remove an FP from the pool.

Well, it's a -1 ability that gives a new skill trapping (or, really, two since it can Maneuver or Block) and a +2 to using it (both of them really). Expecting it to be good in physical combat, too, might be a bit much. At Range is what makes it a combat power, IMO.

Death Pact is rather odd. How is it depicted in the series? Because just letting it give a bonus to rolls would probably work better.

Well, assuming that the Blackstaff is hers, WoJ says that she wants it back, and that it was stolen.

Either the White Council tricked her into giving it up, which she regrets, or it is more useful to her in McCoy's hands.  IMO, there's no way that she doesn't know who has it.