A level 20 unravelling Corpus spell will stop a 'lifting the dangling puppet,'the unlucky target falls to the ground. This whole thing is clear enough to me.
Now for the specific questions:
I want to use sight range, but I cannot actually see the spell, although the effect is clearly within my visual range. Can I target the spell? (I'm not using 'sight of the active magics')
I want to counter the spell before it is cast, and thus before the target is lift of the ground (I have a very high Qui+Fin). Do I have to penetrate the MR of the caster?
The caster has 'deft corpus' so he doesn't use words and gestures. How can I see if he's casting a spell? (would my seeing the manifestation of his sigil be in time for instance?)
Sure you can, you have Line of Sight to the spell (although it's not something physical and visible) and its effect.
I'd say no penetration necessary. You just need to follow the rules for Fast Casting.
That's tricky, reacting to his Sigil would be even harder Fast-Cast wise, and you still have no idea what he's casting and what the target is. It'd be almost impossible in my book.
@Ulf : if you know somebody has a deft form, just guess that form and use the highest unravelling of that form you can manage.
Or in other words: if you have advance knowledge, the stock market is a lucrative place.
Hmm - point. Mind you since the unravelling spell needs a spell to target, you need to wait until the spell has been cast... I'd probably not allow that as a fast-cast spell at all (Rather you'd need to cast one of the variants that reduces casting total for a caster - which does need to penetrate).
In the spontaneous casting chapter, there is mention of the fast-cast defense. It's been raised on this forum aswell. An example of that would be to conjure a stream of water to stop a pilum of fire.
If that (Creo of Rego) Aquam spell can be cast fast enough to stop the excisting PoF, I'd say that an unravelling Ignem spell would be equally usable. IMO all fast-cast unravelling spells should be able to counter a spell. (if you can handle the -10 on casting total and beat the other magus' initiative with your Qui+Fin)
A Cr/Re Aq spell doesn't need a spell to target. The unravelling does. But you're right - you probably should be able to use it. As long as your casting speed matches (not exeeds, because then you'd cast without a target) the opposing magus. It requires luck, but then some people are lucky...
Note what the book says about requiring the cooperation of the opposing magus for MuVi spells. This is similar, in that you want to affect his spell just as it has been cast...
That would be the same timing elements involved in any fast cast defense IMO. It shouldn't be any more difficult. I would think it should be easier actually, with a formulized and mastered effect.
There should be no problem with a fast cast spontaneous Unravelling being effective, if the caster has enough skill. A better option would probably be a mastered Formulaic Unravelling, with the Fast Cast mastery ability.
The real trick is knowing that the caster is casting the spell at all. If you have a reason your SG accepts, you are golden.