without mystery, in items (including talismans) can only affect other effect in items
with mystery: in items can affect other effect in (other) items and spells
each MuVi must exist for a "form" spell target (so for 1 MuVi effect, if you want to do it on all spells, you need 10 versions)
each MuVi has ONE effect: example, you can say that the MuVi spell boost range. This is a minor change (guideline 2 on 3 i think). But a spell canno't do: boost range or duration.
Thus, for the minor change, formulaic versions of spells could readily exists (the many wizard's boost, range, duration...).
For the "major change" (MuVi spells affecting spells of half their level +5 levels), you have the "fork" or "mirror" example. But if you want a MuVi spells affecting, for example Vim spells, of level 50 (45+5 touch) to modify entirely a level 25 spells of Vim form, thus, in fact doing big spontaneous spells without having the arts, you need 1 spell by "effect". So formulaic MuVi in this case is really hard, and spontaneous MuVi is better, to be cast on formulaic other Vim spells.
Example: your spell is a ReVi 25. You don't have Corpus and you want to fly.
With the guideline major change you can: change form, technique and up to 2 magnitudes in power.
But you will need to cast a MuVi spell (level 50) "Transform the (Vim) in Flying" to affect your ReVi 25, success a concentration roll, and you can change your ReVi spell to a ReCo up to level 35 (here, flying could be a ReCo 25 spell only and you wouldn't need the +2 magnitude boost). And, if for any reason, you can't cast the ReVi 25 but have to cast a ReMe 15 you will need to cast: "Transform the (mentem) in Flying
But, what if you wanted to teleport? You will need the same mechanics for the MuVi, but this will be a "Transform the (Vim) in Teleport" spell.
And if you wanted to see in the dark, it would have become a InIm 25 (IIRC) "Transform the (Vim) in see in the dark".
Conclusion: MuVi is a field, interesting with formulaic, but you need spontaneous spell to be really effective with it.
No, that's specifically about the "other magus' penetration total" it doesn't apply here. It also isn't necessary if the two casters are cooperating which seems also to be appropriate in the case of an enchanted device (although I can imagine an argument being made the other way).
Hugh in Magi of Hermes has a muto vim spell in his talisman that I think is a fine example.
A spell/effect so powerfull or Continuous gives Warping. A spell designed that is powerfull doesn't, or if it is continuous less.
Then, i am asking what Muto Vim guideline is more accurate to make a spell doesn't designed to a Target designed to one Target.
I mean, that is a change more important that the Sigil but near to change the Duration/Range/Target or not?
If i understand, you want to transform for example "teleport (designed for myself) level 35" to "teleport (designed for you) level 35". I think it's the same extent of change that the wizard's boost/range/duration spells.
Can you as a magus use a MuVi power on a power triggered by your familiar that is within your familiar bond.
Say, you have a phoenix. You have a bond power that wreaths the magus in flame at the phoenix's command (flame that does not burn the magus of course). Could you cast a spell with MuVi that could in some way enhance/tamper with the bond power?