Welcome!
The two most popular theories Ive seen for Cider Spires are basically the same idea: either it's a post-apocalyptic Earth (Dresden or otherwise), or some think it's a post-apocalyptic Alera and the crystal tech we see is some sort of evolution of Furies. In addition to what you mentioned there is mention of animals that are thought to be mythological (lions, horses, etc) as well as some very earth terms (and cultures by the sound of things) for the different spires.
Personally I dont think it's outsider because I think their rule would be more...dramatic. And I dont really see them just leaving these Spires alone. My money is on aliens. The real question is how many different /kinds/ of aliens? The books mention a surface dwelling Enemy (You there yet? It's not really a spoiler), but there was also that early mention of sailers spotting an "Archangel"
What follows is what I /hope/ for, but cannot really expect:
Dresden is a series focused on Human forces. Alera introduced us to a galactic threat in the form of the zerg vord, so maybe (just maybe) this story of with all it's crystal energy and strange beings are to be the Protoss of some giant shared Butcherverse, and years from now we'll all be reading a huge interstellar cross-over tale that is what Starcraft wished it could be.