One thing:
This is not true. Cleric Stance is the same at any level, on cnj/whm/sch. It is never enhanced by a trait.Also, Cleric Stance in my concept would not be needed at low levels, as it only swaps INT/MND (no damage boost)
Other than that slight nitpick I like the idea. As long as the fields don't look like what GEO put out in ffxi at first. Literally gave me a headache the first day I had to deal with it.
You would have to deal with the fact they'd still have cure/cureII/cureIII/esuna as a dps though, that seems a bit much, especially for a job with what you propose as mp restoration mechanics. If you could limit their MND stat somehow this would work; it'd require them to cleric stance to have high mnd, but then suffer the -20% cure potency.
Edit: Oh, re-reading I seem to have missed where you proposed removing the damage boost on cleric stance. I don't think that's a good idea; none of the arcanist abilities change between smn/sch. Any ability that does change over the course of a job is done by traits to enhance it. Having an ability @ cnj 1-30 then removing part of it when switching to this job doesn't work well.
Give elemental harmony an effect of lowering cure potency could do the trick. Then you'd have a job that flows from higher damage + lower cures seesawing to lower damage for higher cures.
As far as your endgame comment about cleric stance, that's already dealt with, considering the limited amount of i70's with both int and mnd and complete lack of i80+ with both. The only i70's this job could equip with mnd are rose gold circlet, and accessories, which are already available to smn and don't make them much of a healer. Then you're just back to somehow eliminating their use of cure 2/3.
Another thought, make this "elemental harmony" a stance which changes Cure II to quake, esuna to flood, and cure III to tornado, similar to how arcanist summons get modified, and then you'd free up 3 more abilities to come from 30/35/40/45/50 job quests. A similar precedent to that kind of idea is in crafting: Inner Quiet starts out as a buff and gains stacks, so it wouldn't be hard to make a buff that starts as a permanent stance and also gains stacks and loses stacks.