Moose pretty much gave my thoughts on how I think Lahabrea could still be alive: Unlike the Sahagin Priest, Lahabrea was abosrbed into Nidhogg's eye which still holds its aether, while when we defeated Leviathan all of his aether was dispersed. Between that and Lahabrea also being far stronger than the Priest he easily could still be alive, though personally I don't think he can do anything until Nidhogg is defeated again. Unless it turns out Eistinien really is in full control and just trying to bring the end of the Dragonsong War about in the only way he knows how.

On Gaius, I personally don't see him coming back in XIV, though Dissidia is fair game (and if any villian from XIV was brought in, should be him). A part of me would also rather him stay an antagonist, even if he ends up not being "fully" with the Empire. As in he's still gonna try and conquer Eorzea because he thinks it is best, but he might also end up gunning for becoming Emperor, most likely because he believes Varis's goals will bring about the ruin of the Empire. But thats only if he is alive.