As said, this causes lots of issues and consistency problems.
Something similar I suggested, but didn't have the possibility to try, is to have PM be to magic what sunlight is to vampires: It destroys the weak ones, and repells the stronger ones.
That is, have PM cancel low-level magic (as in your A), such as under lvl 10, and have it repel the rest (as per RAW). It still creates some problems but is clear, consistent, and also eliminates the "pink dot" problem. Note, though, that I've yet to see someone attempt a pink dot, so House Ruling might not be nescessary, depending on your players.
Funny, suggested almost the same thing on these same boards. That is, when multicasting, consider that a single spell is cast, with each copy increasing its base level by one magnitude when applicable (ie mostly on Creo/Rego damage spells, PeCo and might strip spells).
While we're at it, if you (rightly) fear Might critters are too easily destroyed by might-stripping spells, you might be interested by the idea of adding a measure of MIght Soak