First of all, I don't think a fixed spell is supposed to be treated like an advantage,
Fantasy calls it a meta-trait only, and with all the changes you need to make to the advantage, I have another suggestion -- Payload or Jumper. But, for academic purposes:
Q1: As I've mentioned, I don't think you should treat it like an advantage at all and you shouldn't put any enhancements on it. Should you do,
Powers suggests going with the
cheaper approach. But I'd say you should buy high enough level of skill with the spell with a limitation "for energy discount only". And yes, it means a
huge skill level.
Q2&3: Speaking of not treating it like an advantage:
PK suggests aiming for a net +0% modifier while changing spells, and to ignore all the Fatigue-related modifiers. So you could balance between Reduced Duration, Reflexive, Reduced Time and maybe Cosmic (see further), to get a net +0% and this could be fair.
Q4: Yeah, I think it's fair to switch an inherent modifier into a different one with the same value. It's pretty much how clerical spells work: they exchange mana dependency for pact and sanctity dependency.
Q5: I'd say Reduced Time is fine for casting time, but to get rid of the forming time is to get rid of a serious inherent restriction, and I'd say it's a +50% Cosmic modifier.
Academics aside, I see two alternatives, both with the leading thought of creating your own dimension:
1. Payload (a bit dodgy) with the Cosmic from
Powers, under Snatcher, and some other Cosmic enhancements that would let you hide in your own Payload.
2. Jumper -- with some enhancements to make it reflexive, have limited time, and reach only one dimension -- your own side-dimension, where you can't influence the outer world and the outer world can't influence you. I'd definitely go with that one.