You are saying that a Muto Vim affects a spell created. Which is not possible: MutoVim has to be cast on spells before they are created.
And something "before its creation" doesn't exist (you can't target Bismark with perdo corpus). So the target of MuVi is not a spell (it doesn't exist). It's magical energy, which ends when the spell is cast, after the effect of MuVi, and thus, the spell is perfectly natural.
The spell being not the target of the MuVi cast before its existence, but only the result of its application to its target (the magical energy worked in a spell during a process called "casting spell"), the MuVi :
- is momentary (however if your casting was ritual or ceremonial, it would have had a duration) in case of formulaic
- has to penetrate the penetration of the non cooperating caster (not the MR : magical energy is external), because penetration is the "strength" of a spell, and your spell has to force the other caster to affect his process of casting.