Spell Deflection

That the sentence

is not IMHO any solid argument. THat a thing has not been changed through the editions does not mean it has actually been reviewed and actively and deliberately included for a good reason. There is plenty of wierd legacy stuff still around. And I see no reason or argument in Hermetic Magic Theory why Muto VIm shoudl work differently that Rego or Perdo, why it should be limited to only this tradition.

However Muto Vim is fundamentally different than the Rego and Perdo: Where Rego and Perdo Vim have "binary guidelines" with either a universal-but-hard or a very-specific-but-easier way of deflecting or dispelling either works or it doesn't.
Muto Vim guidelines instead differ over the size of changes, and has three different guidelines for this (greater changes needs Cr or Pe req), and they don't mention whether spells need to be Form-specific at all. But the example spell Mirror of Opposition is Form specific, while Shroud Magic, Sorcerer's Fork isn't. Wizard's Boost is Form-specific (and don't get started on the whole "Boost".
If Muto is going to work in the same way as Pe or Re the guidelines need to be worked through, with a universal versus specific distinction. And a spell like the one I suggested earlier should numbers-wise be the Form/Tradition specific version and the universal version needs to be significantly harder - like Winds of Mundane Silence is harder than Unravelling the Fabric of [Form].
And this would make Muto VIm spells to return a spell to the caster really, really hard as formulaic since they will either be high level or so specific you'd need a whole array of spells in order to eb prepared. Or you'd need to do them Spontaneously - which is also very difficult.

The easiest thing would be to simply accept Muto VIm as a way to boost own spells and let sleeping dogs lie. And if facing a hostile wizard instead deflect or unravel, or simply rely on a good Parma Magica and learn offensive magics.

I wholeheartedly agree that this concept is so difficult and needing so much specialization that unless it is a major focus point of the saga the character is severely limited in all other areas.