"Ah! Good questions. My family's particular affinity handles the incantation replacement ... partially automatically. We can cast spells from our own tradition without having another incantation in mind, but trying to adapt a spell from instructions for another tradition goes better when the incantation is already known and you keep it in mind. For the list of elemental affinities — the most common ones are fire, earth, air, water, wood, metal, light, and darkness, in rough order of commonality. But you're correct that there is a long right tail; I know of no reason in principle why someone couldn't have an elemental affinity for, say, non-magnetic steel alloys with high amounts of molybdenum."
"Giving the same kind of answer for schools and traditions is a lot harder, just because many of them are secretive, and don't share details with outsiders. I can say that I have personally met Christian priests, Caddo shamans, Vodun practitioners, street magicians, demon cultists, new-age ritualists, and raw elementalists, all of whom practiced some magic in a way familiar to me, and some in a way totally foreign. With enough experience, you can often pick up enough about magic-in-general to at least infer what someone from an unfamiliar school is doing, but assembling a comprehensive identification guide would be the work of many lifetimes, if it were possible at all."
"And that great diversity and secrecy is why I recommend learning Thaumaturgy first: I don't actually know of a way to test for any other affinities, outside rare circumstances. For example, being able to burn demons with holy symbols is a sign that one is well-suited to Theurgy — but running into a demon with which to test that is usually not a good idea. Anyone can learn to practice Thaumaturgy, with enough care and time. Ease of casting elemental Thaumaturgy spells is the easiest test for elemental affinity, besides just going around and trying to shape every element you see with your mind. What exactly makes one suited for other schools?"
He shrugs.
"Who knows?"