As for magic in SSSS-verse...
I have a theory I've stated on the forum some time ago: all that Rush-thing was to open the doors to the Collective Unconsciousness, where all the gods were actually created by the power of men. There are few additions to it now:
1. Humanity gave a lot of "energy" to the CU, but it also can be drawn from. And as the number of humanity dropped sharp there's a lot of "energy" to a few humans.
2. An ability to do magic might actually be a symptom of the Rush's alternative strain (or was it the original one?). A strain of a fungal infection that is only transmitted via amniotic fluid or sperm OR transmitted in any way but contagious for an ambryo only.
3. Thus the Kade phenomenon might be viewed as mages body becoming a field for battle or symbiosis of those two strains.