Depends on the level of the spell. Art & Academe offers guidelines that cover either case. Furthermore, such a spell may very well allow a natural resistance roll.
Creo Ignem and looking for shadows. A combination Creo/Intellego Ignem may very well do both for you at the same time.
Yes. The species are changed and destroyed. In the end, they are destroyed.
The mask. The argument being that the MuIm spell changes species as they get created. Whether or not they get destroyed doesn't cancel the spell. The other way of looking at it would be that since the species are destroyed, the spell stops having a valid target and thus fizzles out. But this is proven wrong by Perdo X spells with a non-momentary duration (which keep on destroying if you throw more X at them), as well as the fact that spells active on Bjornaer magi carry over when they change shape, even if their new form is not covered by the spell's form. Therefore, the MuIm spell is still active when the PeIm spell ends, and resumes transforming species.
I have no idea, though the distinction of whether he was dreaming or not (and thus whether his mind was "busy somewhere else") may be a key issue.
InIm won't work for just the reason that you stated. However, InAn will detect an invisible animal, and InCo will detect an invisible human. Basically, you detect the thing itself instead of the species that it would normally emit.
The difficulty with this sort of detection is you have to penetrate any magic resistance of the target before it can be detected. This is not normally necessary when just detecting non-magical species...
Sure it will. Just dont try to "aim" it at detecting what isnt there.
RAW invisibility always causes a shadow, so you could for example make yourself see shadows perfectly. Or you can look for the "hole of species" by amplifying your senses...
Ah, but you're not detecting the invisible object directly. You're just inferring its presence by its apparent effect on the surrounding environment. It's not (quite) the same thing...
It is actually possible to become invisible without casting a shadow. It's just much harder. A constant ReIm effect could do the trick (by causing the species that would be blocked by your presence to avoid you) or perhaps PeCo (to destroy your ability to cast a shadow...). Most people wouldn't go to this much trouble, but it is possible.
Well, given that a CrIm spell cannot create species but must instead create an entity capable of emitting species (otherwise they'd be magical and magi would automatically be unaffected by illusions), I see no reason that an InIm spell couldn't detect the capacity for creating species, or for detecting the process of altering or destroying species.
To continue examples, InIm can also be used to detect someone invisible through smell or sound, as they arent affected by normal invisibility spells.
Then of course, you can use InAu to detect where there is a "hole" in the air, InAq to see concentrations of water, InTe to feel where a person stands/walks(provided target not also using flight of some kind of course) or where there are metal items above earth(belt buckles, weapons etc), InMe to find out where sentient beings are nearby etc etc etc...
Thats just the beginning of all the spells i have seen through the years used for detecting invisible people.
Invisibility is good against non-magic users, or anyone unable to use any spell good for detecting them, but most magi will have decent ability to find invisible stuff.
Your constant ReIg should be to pass light around you, otherwise you still create a shadow, possibly even making yourself look like a shadow. I would suggest a MuCo spell to make you transparent instead. I dont think you can destroy a "negative" ability, like you cant prove that something unknown doesnt exist. A PeCo spell for removing your scent would be a good idea however. Or a MuCo one to make it smell like something natural in the surrounding area. Another PeIm spell to remove noise from around you, a InIm to hear normally "through" the noiseless area around you.
And, finally a MuVi spell causing Intellego spells to either misfire or show something that isnt the real thing.
Oh and a PeVi spell "on hold" that activates and tries to dispell any PeVi spell that attempts to remove or alter any of your spells.
NOW weÂ´re getting close to invisibility against magi as well...
shadows aren't created by Imaginem (although you see the shadow by the species the shadow emits).
shadows are created where the Ignem light emitted by a light source is blocked...
(Hence you get the interesting effect of a magus (with Parma) in the presence of a magical light source - the Ignem light spreads around and hits most things, inducing them to emit species, which (being natural species) the magus senses.
The magus themself has Parma, and blocks the Ignem light (a magically created medium) so stands their in a pool of shadow wrapped solely around the magus...
You can add a complex ReIg effect to make light bend around you and carry on past;
or a MuCo(Ig) to make your body pass light unhindered.
Or a complex CrIg "fill-in light" effect to illuminate the area of any shadow you cast by blocking other light...