What happened to SB (story)?

[CosmicKirby]

I apologize for misreading your statement; it was an oddly worded one and I read over the seeming redundancy (the son of the spawn of), however it doesn't change that Bahamut and Nidhogg are, as it were, the same tier of being (son of Midgardsormr). It's also possible the ascians gave the eyes another refill once the Warriors of Darkness delivered them.

This is an extraordinary assumption that requires explanation. Coming up with answers, for the authors, is not your job, it's 'theirs'. There's not even a hint that the Ascians are capable of this, and if anything it's implied that it was purely Nidhogg's doing on his own. That he, for some ungodly reason, could remain alive through his eyes alone.

We also don't know how spent they were--they were dangerous enough still that keeping possession of them was dangerous and could get us possessed. Dangerous enough that we had to use all our power to rip them from Estinien still. Dangerous enough the Ascians and Warriors of Darkness considered them worthy to be enough to summon a dread primal with power akin to Bahamut, which later turned out to be Shinryu.

The only time we see the eyes completely drained is when Estinien declares the balls inert and stabs them. But even in their drained form, they were a threat, maybe because they still gather and generate aether over time. I mean, one of his eyes has been in the possession of the Azure Dragoons for a millennium.

The fact that they can be drained 'at all' as stated by Estinien calls into question how such a mechanism works. One in which is never really answered. One would SUPPOSE that powering 13 beings into primal status simultaneously, absorbing an ascian whole sale, and reconstituting the entire body of a being not of this planet would amount to some sort of strain on them. But apparently, not. Simply summoning a primal from a man's imagination though? Somehow too much. There's no attempt to state what the eyes are capable of or how they work beyond being magical until AFTER the plot is done with them.

In addition to his insane age, being one of the First Brood, Nidhogg is also one of Midgardsormr's direct sons (clearly). As a show of his power he turned an entire species to his cause and waged a 1000 year war on his rage alone. Rage that overtook him and became a completely different beast, easily capable of defeating Hraesvelgr (another of the First Brood) in single combat.

EDIT: Edited to highlight an explanation that might be satisfactory.

Clearly Nidhogg is so powerful, that'd he never lose his eyes to mere mortals. Or lose against the Warrior of Light, twice. Or let the Azure Dragoon pierce his skull with a spear. At least Zenos was demonstrably capable. Nidhogg certainly wasn't, and yet his eyeballs turn into an overused recurring plot device. (Much less, 'his' eyes in particular. As no mention of Bahamut's eyes, Ratatoskr when slain couldn't reconstitute and her eyes were so weak they could just be eaten, etc.)

Cosmic Kirby, there was a quest where we tried to prevent the kobolds from summoning Titan again. We were even warned by a kobold child that they were getting the crystals and materials ready for it.

During the entire process that child came face to face with its dead parents and in an explosion of grief summoned Titan using the aether of the nearby gathered crystals by pure accident.

Accidental summoning has been a thing since ARR/HW.

Assume that I have actually played the game please. My primary point remains that it's a wonder how the humanoid races of Eorzea have (Up until Shinryu, and to an extent Shiva) yet to summon primals.
(And Ga Bu wasn't even entirely an 'accident' The ritual and all the trappings were in place. We THOUGHT we stopped it, only for Ga Bu to complete the ritual himself. Therein was the accident.)
(Further, I am struggling to recall any 'accidental' summoning in 2.0-2.5 Ifrit, Titan, Garuda, Levithan, Ramuh, and Shiva were all intentional and underwent ritual and religion of some sort to summon. At least, the FIRST time if you want to ignore when they just resummoned to fight Ultima weapon in an instant.)

[Daralii]

At least, the FIRST time if you want to ignore when they just resummoned to fight Ultima weapon in an instant.)

Ultima didn't resummon them, it had them contained in the Heart of Sabik just like Bahamut was contained in Dalamud. The Heart's a big mystery, as Lahabrea(I think) said it was pre-Allag and only a fraction of its power was unlocked, but it's never been mentioned after it was used as a plot device.