CLOK BBS

The four remaining prime immortals are Iarel, Undm, Vandin, and Vodr. Only two of these immortals have any kind of direct relation to elements, those being Vodr, water, and Vandin, fire. The other two require a bit more indirect theorizing. Undm is associated with death and rebirth, so I would say they are more closely associated with earth. Besides graves are in the earth, so that seems to make sense. The one that is only maybe a bit of a stretch is Iarel, which would have to be air. Given her description most closely resembles that of Khaldeans and her strong ties to Nuum, I think it's safe to assume that Iarel is most associated with deserts, which are dry and arid, so this too makes some amount of sense. For Undm and Iarel, there is not much mentioned elemental wise. In fact, Iarel is only really mentioned for granting occult knowledge, medicine, and poison. These aren't really areas that coincide with air. However, aeromancy is one of the weakest elements in terms of direct damage, so her being more associated with healing makes some sense. So what does this all mean?
I believe that elemancy draws directly upon these four immortal's domains. I don't really know if it is channeling these immortals, or just using the same powers that they command. I'm leaning towards the latter, personally. Channeling elemancy doesn't really require anything other than the aptitude from what I recall. Each student is limited to mastering only one element though. I think maybe the elements themselves are cosmic powers that these four entities managed to master, and that might have been their first step towards unlocking the secrets of the universe.
At any rate, it's a theory I'm still working through. Some of the other immortals have elemental ties, but it is not clear as to which ones belong to which. Especially when looking at the druidic immortals like The Great Bear.

Would the concept of elemental mixing imply that there are other personifications of mixed elemental heritage, possibly "immortal" but potentially more of a minor supernatural being, for things like steam, acid, lava, etc?

It is something I had thought about. Maybe at one point there were prime immortals that could channel acid or steam directly without needing to weave elements together. According to Vodr’s wiki page and lore, there were many other prime immortals. It’s possible that these were masters of these other… I don’t even know what to call them, weaves? Patterns?? With their loss, the four remaining prime immortals had to get creative. They knew these other weaves existed, but could not directly channel acid or steam. Maybe they taught one another the basics of what they knew to accomplish these weaves. Elemancers can learn all four elements after all. However, something prevents them from learning more than two journeyman and one master. It could be a mental limitation, but I like to believe it’s a cosmic limitation instead.