So, that's a lot to take in! I admit I had a rather simplistic view of what a primal was. My understanding based on Hraesvelgr's lines was that a primal is essentially a massive aetheric construct that's imbued with a form of sentience because it's summoned with emotions and memories of its followers. (Arcanists, on the other hand, summon carbuncle using a series of mathematical formulas). In the case of a primal, the memories and emotions of the summoners are "given life" by the aether, which is why primals have personalities and memories, unlike a carbuncle. I also see a rather cyclic nature to primals. The more a primal is summoned, the more definite the form become, the more the aether use to construct it is influenced by the primal until it becomes an elder primal which no longer needs the beliefs of it's followers to take shape since there's enough aspected aether in the life stream. I may be wrong in all this, feel free to correct me!

The primary difference I saw between Ysayle and Thordan was the Echo. I took that to mean Ysayle was able to keep her aetheric essence from becoming intertwined with Shiva, allowing her to change back and forth from a primal. With Thordan VII, my understanding is that his "ascension" was similar to Louisoix's, meaning that his aether is now intertwined with Thordan I's, so you can't summon one without the other. The thing that strikes me as being different about these primals and the beastmen primals is that they were summoned into sentient, living beings, so the resulting entity is mixture of the formerly mortal's personality and the personality of the newly created primal. Garuda, Ifrit, Bahamut, etc. were all created from the memories of their respective summoners, which is why they tend to be a bit more single minded.