The headband is less important for anything to do with spells - its for buffing INT skills to +3.
Lae'zel as EK or Astarion as AT shouldn't be relying on INT, though you probably do want 14 int / wis / con on Astarion for investigation and perception.
For the spells if you want damage on those subclasses you just use magic missile and witch bolt, and both are rarely needed as your damage comes from weapons. The best use for the spell slots is Shield, Mirror Image and Blur, and optionally the movement buffs like longstrider, but I find those to be a waste of spellslots. If you pop a shield react before taking multiple hits you block insane amounts of damage.
I think I used it for my Shield Dwarf Wizard in EA and it worked for spells. Does that not work anymore?
