The dispel magic one is probably working as intended. That ability is a combo detonator, and using it on frozen/paralyzed enemies triggers an ability combo (the affected enemies take some spirit damage from the combo). The AI in this game will try to detonate ability combos at every possible opportunity. Yes, it has the unfortunate side effect of removing the CC from the enemies, but the AI can't know when you want to trigger combos and when you want the enemies to stay CC'd, so it always goes with the more aggressive option.
As for AOE abilities being wasted on single targets, that seems pretty par for the course for game AI these days. In DAO and DA2 you could improve the targeting with tactics, but that's obviously not an option here. Just another unfortunate drawback to not having access to tactics, I guess.