Idea for a plot point

[Cilia]

Just... ehh... elucidating a few points.

Neither [Cid nor Nero] have a lot of reason (or desire) to go back to working for Garlemald.

What led to Cid's defection, specifically, was the incident at Bozja. Aside from it claiming the life of his father (who could be considered semi-tempered by Bahamut at that point, or at least so obsessed with Meteor he may as well have been) it was Solus' willingness to continue the experiments and weaponize Meteor despite the risks it posed that led to his defection. His conscience simply couldn't abide what Solus wanted to do, and he's remained an expatriate due to disgust for the Empire's imperialism and brutality (another factor leading to his defection was the conquest of Ala Mhigo which, while quick, was supposedly incredibly brutal).

Nero can't go back to Garlemald even if he wants - since he was part of the XIVth which went rogue, he's wanted for desertion and that's apparently punishable by death. He joined the military only for the opportunity to get up close and personal with Allagan tech since engineers only got the scraps the military didn't want. (Despite his outward disdain for the Eorzeans' simpler lifestyles, he seems happier as an itinerant engineer in the region.)

Apparently, the Battle of Silvertear Skies did... something. Whatever that that something was it stopped the XIVth Legion in it's tracks and prevented them from going into Eorzea for the next fifteen odd years. What that "something" is, we don't know. We do know it has something to do with Silvertear Lake though. That said, the only reason why we know any of that is because of Koji telling us that's why the XIVth Legion didn't resume invading Eorzea until Nael was handed the Metor project.

The point of invading Silvertear was to gain control of the aether in the region. Midgardsormr threw a gigantic monkey wrench into those plans; not only did he halt the attack, his death made it possible to summon eikons in the first place (no records of summons exist prior to this event). Nobody's really sure what kept the Garleans holed up in Ala Mhigo for 15 years, but it's plausible Gaius was simply being shrewd and not attacking to prevent those summons. He refused to march on Eorzea again until he had a way to permanently deal with eikons, having the foresight to recognize that attacking would simply cause the same summons he wanted to prevent. (Meteor being the nuclear option would make them a nonissue in the end.)