"Sustain or suppress a spell cast by another with level less than half the level + 5 magnitudes of the Vim spell" or the other one for your own spells ...
I'm having doubts here. Looking at MtDS. The spell has a duration of diameter & simply allows concentration spells to be maintained for a diameter keeping the original effect within the same power level ( Concentration == Diameter ). Are we pushing the guideline by allowing sun durations of MtDS? Does it then fall into the MuVi realm to boosting the spell?
MtdS sustains a spell that you have cast yourself - so it does go by another Guideline.
But I don't see a reason to forbid sustaining the spell cast by another for longer than Diam with ReVi. Control beyond sustenance of that spell is not mentioned or covered by that Guideline - and that it where MuVi Guidelines come in.
With the exception of the Momentary duration spell, I don't see a problem.
One's unlikely to find a Momentary duration spell with an appropriate effect that can be maintained. Supposing that there is such a spell, I'd require both spells be cast simultaneously, without the benefit of the reduced ease factor on MtDS, because the spell it is to maintain isn't actually being concentrated upon.
I'm reasonably sure that the 'sustain a spell' effect only works on D: Concentration spells.
If you have a D: Diameter spell that you want to use as a D: Sun sun spell, modify it using MuVi.
This has been debated somewhat though.
However, you can design a 'sustain spell' effect with a duration other than D: Diameter, easily enough.