edit - Lalwendwe has already efficiently posted some points I was bringing up...
Davem,
Your correct. There is too much out there to unify it all. There is also the distortion that history provides that muddles up the picture. You have mentioned the Sidhe, and previously the Tuatha De Dananns. My problem is where to stop in regards to Tolkien. Do we stop at Celtic/Nordic traditions in regards to faerie?
There are other subjects (dragons come to mind) that cross cultures and history. In that regard, fairys (faeries) have as well: Greek, Arabic, Egyptian etc. You have your muses, genies, sirens, nymphs .. the list goes on.
What I do notice in all these variations is that Faeries are not divine in and of themselves (although most are immortal), they do represent (in most cases), the transition, or go-between our world and the "divine" world. Either as messengers, couriers, propheters, or just because they can, and want to.