Iris Havengard Current Progress

I think that idea is pretty cool thematically, but I think it will be fairly useless if the attack strength isn't bumped up a notch.  I'm assuming for the moment that it's highly unlikely she'll join before the Magitek facility.  It would be a lot more work and it would make zero sense with her story arc.  That means that by the time she's recruited, she'll have access to "mid-range spells".  Assuming that the "mid-range" spell for Ramuh is Bolt 2, for instance, 3/5 attacks are on par or strictly worse than what she could spam every turn, the fourth is an overflow with drawbacks, and the last is just the Esper Summon (which isn't great in this game).  It can't even really be useful by saving MP, since MP can easily be replenished by Osmose by that point.  The only use I can see for it is that you don't have to learn the spells before casting them.  I think that she'll need to be casting Bolt 2 and Bolt 3 (even in the WoB) instead of Bolt and Bolt 2 for this to work properly.

In a normal hack my solution would be to rebalance Osmose so it isn't broken, but that won't work here.

I also don't think this will work in the WoR at all, even if the backfire is removed after her sidequest.  Once you can spam the most powerful attacks at will, it's hard to justify using a move that will basically just use similar attacks sometimes.  That is, unless the overflow attack is still worth it, or you don't teach Iris every spell.  It might not be a huge deal though, as a lot of the skills slip into niche or outright useless roles by the endgame when compared to Magic, so Fuse would still be middle of the road in terms of usability.