Like a massive object Creo'd right above someone's head - that discussion has gone the rounds as well, hot and heavy at times.
The one major pitfall with your approach is that "creative" players can do great damage with relatively trivial spell effects. Once you try to split hairs between "damage" spells and other, the door to headaches is thrown wide open.
Here's one such thread, from earlier this year...
Just for the record, and so no one confusing B's personal position with canon, this is from the Core Rules:
[color=darkred]The Functioning of Magic Resistance (Core Rules, p 85, col iii)"...Things that are created and sustained by magic (anything not created as a mundane thing by a Ritual spell) are magical for these (i.e. Magic Resistance) purposes.So, while the book fully supports StoryGuides making changes for things that don't work for them, BoXer's interpretation is just that - his, and his alone.
Just to be clear for everyone.
Fair enough, I concede my error. My view was more a matter of noble's parma anyways and mostly for the sake of good discussion. Stick to the RAW.
These issues are very different. This thread is a straightforward issue of penetration, going back to putting pink dots on swords to have parma save a magus and the like, well versed in many places.
Suffocating (or breathing) and magical substances is less explored.
I started reading the topic mentioned, and you don't even have to finish it to get the answers: an arrow created by non-ritual magic is affected by magic resistance.
For simplicities sake I would rule the entire 'split-arrow' attack as magical, even if there are a few non-magical effects involved.
I think the previous discussions here and the examples in the book have clearly shown that creating a large boulder over someone's head will bounce of Parma, leaving the magus unharmed This should also be true for an arrow, created magically (even if it is Muto). The speed such an arrow may or may not have is not an issue when it comes to penetration of Parma (it is of course an issue when it comes to penetration of grogs).
See The Crystal Dart MuTe10 for a very similar effect and add Group and +1 size.
It is a Muto spell like your wish.
Lowering the range to touch and raising the base effect to 4 (metal) don't change the level. It would change some dirt into arrows.
The text clearly states you need to penetrate the MR.