Some interesting and plausible ideas here. Although i'm beginning to realize that summoning rules as a whole are.... flimsy. Which is somewhat understandable, given the restraints that the original "rules" put in place. After all there are other instances of summoning against the rules. For example, Garuda managing to simultaneously summon Titan and Ifrit by inflicting pain on a very small group of individuals with no crystals in sight. Then Ysayle summoning Shiva solo whilst in free-fall (Which MIGHT have possibly been powered by Hreasvlgr, although nothing actually made this distinction). Also, was it not meant to be crystals of the correct elemental type? If so then that idea has gone out the window with King Mog, Ravana, and King Thordan having no discern-able elemental type(Although i guess the eye could technically hold all elemental types).
On a final note, how important are crystals anyway? Bahamut was imprisoned for millennia, yet there was no sign of crystals in the coils. It's also been mentioned that many other Primals like beings have been captured by the allagans, do they imprison each with a life supply of crystals?