Circlets and Helmets have the same enchantment slots, so any enchantment that works for a helmet can work on a circlet. And to answer your second question: Morokei, a mask identified as a circlet has +100% macgicka regen. Morekei can be acquired from defeating the dragon priest Morokei in the Staff of Magnus quest in the College of Winterhold storyline. Morokei (the dragon priest and mask) is in Labyrinthian ; the location is locked, up until the a-fore mentioned quest
You could use the Aetherial Crown and have the Apprentice and Atronach to give you a whole load of magical bonuses. While you would have take 200% damage from magic, the Atronach still has a 50% chance to absorb all incoming damage from a magical attack, so you take on average the same amount of damage as if you had neither stone. If you also pick up the Arch Mage's Robes, you can wear the crown with them for a total of:
200% boosted Magicka Regen
100 extra Magicka
15% reduction on all spells
100% spell absorbtion