I think it's still bugged at the moment. There are supposed to be maintenance cost, you can see all the permanent spells listed in the "Enchants" section of your sovereign/caster when you click on them, but I do not really see any penalty to your maximum essence.
Yes. And for what I saw, there´s a maximun mumber of those you can have at any one time. Like, if you have too many buffs (city and personal), you won´t be able to summon more units until you free up enchantments.
