Given the many spanners thrown into existing theories about primals, I don't think many of us can confidently claim to know everything about them any more.

First, Ferne has dropped a major hint at the Las Vegas Fan Fest that primals are not necessarily elemental.

And I don't recall anyone ever saying a primal drains or strengthens an element. The only thing we know for sure is that they are sustained by aether and strengthened by worship.

I still need to check whether it was the primals or a vast store of aether (or both) that was sealed away in Silvertear Lake. We don't know who created the seal, although my guess has always been the Allagans. And if it were indeed the Allagans, then for them to have sealed away modern primals like Ifrit, Titan and Garuda would suggest that these primals are some 5,000 years old. How much older would that make Bahamut and Odin? I don't know.

In any case, whoever created the seal clearly believed it was a way to stop the primals from reappearing in Eorzea, so if the inadvertent effect is to cause an "imbalance" in the aether, the rest of the realm does not seem not to have felt it for a good long time. Either the peoples of the world adapted to the new balance, or there wasn't an imbalance at all. Moreover, it has been repeatedly said that the primals are the ones who disrupt the balance, rather than their absence, so I don't think the seal is to blame.

As for the effect of summoning primals in areas filled with over-aspected crystals, I also don't know. I know only that a summoner has to go to a place strong in a certain element when he attempts to bend an egi to his will for the first time. I would think it particularly suicidal for a summoner to go to an area that is too strong in that element for such an attempt.

From what I recall about the Ultima Weapon, it was actually the mechanised version of an Allagan summoner. It was designed to absorb and weaponise the essences of primals, not unlike the way a summoner would siphon a part of a primal's essence in order to create an egi, except on a much grander scale.

And since it's a question of scale, I'm not surprised to see the Ultima Weapon summoning the primals themselves when it attacks. No mortal can hope to have sufficient strength of will to command a full-scale primal. But a mega-machine, on the other hand, probably has no such limitation.