3.1 MSQ Reaction [SPOILERS]

[Tenkuu]

I think Lahabrea came to him right after - when Louisoix cut off the seventh rejoining. The ARR Echo of the Battle of Cartenau suggested that Lahabrea was sure that this is it, baby. Why come to Gaius before that? Once the whole "phoenix" thing happened, I think Lahabrea figured - welp - needs moar chaos - and pulled Ultima out of Gyr Abania's basement knowing that Gaius would be the most likely to embrace gifts of extraordinary power in such a way that he'd immediately use them to wage war.

About that, why didn't Lahabrea go for Omega after Ultima's destruction?

[*] The Garleans attacked Silvertear Lake because they, too, knew of the legends about the place and realized its aetherial importance and knew that it could be tapped for summoning. (Confirmed by Fernehalwes)

So what were they aiming to accomplish by attacking it? Were they going to occupy it?

Same. If you closely follow his story from beginning to end, he was what I call a true believer. He bought into the notion that the Garleans, the second coming of the great empire of Allag, were all that stood between the planet and its destruction - that might is right and all nations must be brought under the Imperial standard by will or by force in order to save them. And, compared to others in the Empire, Gaius acted (at least as far as his principles were concerned) with honor.

I don't think I can call a man who would sanction the slaughter of several innocents "honorable". Not to mention that he essentially made his move against the Waking Sands when the Warrior of Light was otherwise occupied and then proceeded to capture those he did not kill. To me, Gaius is not so much honorable as, like you said, a true believer. More than that though, I think he was a very practical man who, instead of relying on blind faith in some gods who only respond to crystals, instead relied purely on his own strengths and abilities.