Hmmm, from my understanding the 2nd theory is closest to the truth. I say this because of the Primal's ability to temper.

The Sylphs for example summoned Ramuh after the Calamity and this physically and mentally changed them. They are now purple colored and are fiercely territorial and go out of their way to try and have their untempered kin brought into the fold. This all suggests that the Primal changes their worshipers, not the other way around.