* Original post altered to feature a numbered list instead of bullets, for ease of reference to each point.
A quick critique on the relationship between [1] "All GCD heals are now free" and [4] "Healers now siphon 500 MP every 3 seconds when they're not casting":
If you go with [4], you'll also need to determine if this is a server tick or is based on the time since casting, and if the latter, whether progress is saved and therefore accumulates, in which case swiftcasts and shorter casts mean more mana regenerated. Which... is not terribly intuitive. (And if you nonetheless want the gameplay implications of that, there are far clearer ways to make those win conditions visible to players.)
More importantly, though, there's no need for [4] if you just take a softer approach to [1].
Just fixing Spell Speed's MP inefficiency by having MP tick per GCD (a 20+% increase to MP generation) instead of per a fixed 3 seconds, and/or a base buff to MP regen or Lucid Dreaming, would net you enough MP for a sustainable oGCDs even while giving them a cost proportionate to their potencies or higher (e.g., at least a Cure II's cost for a Tetragrammaton).
Else, it would seem more reasonable to greatly reduce the CDs of the oGCDs and spend MP only on them oGCDs and Raise, while similarly retooling other casters to share cooldowns, in a sense, via MP affecting their abilities more so than their basic spells.
_________
Tl;dr:
Rather than seemingly to arbitrarily punish the use of filler attacks and requiring the occasional extra pause of .5s after an instant-cast as to qualify for having "not casted" for 3s, etc., I'd pick either
- a system by which the cost of abilities are essentially their net cost or their excess over the minimum cost per GCD (if you spend 200 MP per average GCD but regenerate 250 MP per average GCD, you're still net positive) and you needn't mess with the base cost per GCD or
- a system by which they're the only cost.
________
The rest seems fine.