Sublimation
I'm fine with the way this works now. If it were straight refresh I might feel differently but Sublimation has that nice added sleep avoidance. Sometimes I don't care for the quicker charge as much as I care for that. Also, since the upgrade to sublimation for SCH main, I have yet to be in a situation where I really want sublimation to charge faster when I can't be in idle sublimation plus mode. I know, do more with more MP, but it's not really something that has hit my wish list yet.

Dark Arts
Sounds good to me, though tier II storms might cover it and I'd like these more for their ability to help other casters.

Light Arts
I look at this a bit differently. I honestly think with Korin Obi + Aurorastorm + Twilight cape SCH is in a great place with cure potency even while being left off of Roundel and not having potency on our specific gear. My problem with main healing isn't so much potency, it's not having another large cure for when the Cure IV timer is down and someone needs it again after I've just filled them up (or someone else needs it). Also, the high enmity from restoring so much with Cure IV. It's the inability to spam and to avoid the enmity spike that bothers me. We'll see what Regen V does. It might do the job, at least on the 3 mobs that don't dispel it

Fusion magic
I've always loved this concept of yours, especially the names and for their encouragement to use Immanence. I also like that the use of these spells would benefit all casters in the party, like storms and Klimaform can. I think at one point you had also suggested being able to create Cosmic Elucidation with Immanence and the light and darkness fusions. This would be an incredible 99 ability for SCH.