I’m happy yet distressed

imo use your summoned eir, +res isnt a bad boon at all. if you have a unit with hp reducing skills/weapons (alm or b!celica’s double lion, atk/spd push, or a fury user you particularly like) you might want to fodder mythical boost to them, otherwise keep her until there’s a unit you would want the skill for. unless you really like eir of course, then you could merge her, but i don’t really see the point of merging units with good fodder if you’re not planning to at least +5 the unit.

+Res is better than neutral.
If you care a lot about Aether Raids, you might want to merge her, if you have a unit that might want Mystic Boost (high merged Fury users, units with weapon recoil damage, etc.) then you might want to fodder.