One of the characters in my saga is a Flambeau demon-hunter. The player recently mentioned an interest in developing a version of Unraveling the Fabric of (Form) which affects Infernal magic, which seems to be possible based on the last part of the spell description. Are we reading too much into the 'non-Hermetic magic' statement? Is it intended to mean other powers from the Magic Realm or supernatural powers from other Realms?
Considering Wind of Mundane Silence can take down "any spell" (which would include infernal ones), I don't see that this would be a problem. You would need to specify more than just Infernal for the specific guideline, though.
Seems fine. Although you will probably need several different versions, one for each "Form" of infernal magic, and you will need to be sufficiently "familiar" with each.
Also, from an in-character point of view, it is probably dangerous for the magus if others know he is so "familiar" with infernal magic. As it's only a short leap from dispelling infernal magic to casting it...
I'll have to plead Noble's Parma, but IIRC one of the books describes acceptable non-hermetic magics as including examples such as: Faerie Illusions or Infernal Curses which is a bit broader than requiring a different spell for each of the Infernal Powers...
The trick is deciding when the magus knows enough to be 'familiar' with the Infernal Arts (enough to dispel them as such anyway). This Link might be of interest to you.
Did you mean HoH:S, p.129? There is a ReVi Guideline, and a few spells (dispelling shapechanging and preventing curse magic) meant to serve as examples for possible width of spells countering non-hermetic powers. Application of these examples to powers described in RoP:I is not straightforward and will need troupe adjudication.