Yeah You have right, maybe that Glamdring isnt best idea because Narya most be included in stance but I think there isnt good to have two thing with same effect, spell book magic dmg bonus + agro stance magic dmg bonus, that is thing of bad gameplay design. One of those two bonuses should be changed, maybe make spellbook power to give some aoe basic attack, because Gandalf the White show his skill with sword in defense of Minas Tirith. But there should be problem because Gandalf gets magic dmg buff in 1. lvl with his stances what is not that great in terms of gameplay balance, but that could be solved with scaling damage of abilitys with Gandalfs levels.
I think some people already point to this thing, everybody will change into agro stance use abilities than swap into deff, and that should be permanent buff to abilities, and that could be problem, technically You did nothing but buff Gandalf or make him a little bit harder to use what is not thing we need in mod. So there should be some cooldown to swap stances or what I already suggested to make his defensive stance stop his abilities to getting from cooldown or something like that makes cooldowns longer.