Elder Primals

[Enkidoh]

Well, let the speculation commence! On the first point, it fits in with the Ascians' MO of teaching summoning - they started with the beastmen, and clearly showed they're now going beyond that and actually teaching members of the spoken races as well (ala, those downtrodden kids in Little Ala Mhigo to summon Rhalgr). Given the fact Lahabrea mentioned that the existence of the Primals is merely a means to an end in their goal to rebirth Zodiark (to create a massive confluence of chaotic energy to overpower Hydaelyn), giving a crazed SMN the power to call upon the essence from a Soul Crystal directly and summon the egi of a long forgotten Primal only furthered the Ascians' goal (spreading chaos and destruction).

As for why Belias-egi was simply a large pallete-swap of Ifrit-egi though, I guess that was just for technical reasons - to save on memory and development time that would have to be spent on making a new enemy model (and that pretty much is conclusive that the real Belias will probably then never be actually fought at a later date).

Unless there is more to that than meets the eye - maybe there's something like FFIX where summons change their form over time depending on how the myths and legends that give them life evolve and change with each retelling. Maybe, Belias and Ifrit are one and the same, with 'Belias' simply being what Ifrit was known as thousands of years earlier, even before the amal'jaa arrived and adopted Ifrit as their god.

But again, this is just all my speculation so take what you will of it.