The Real Life Inspiration For Emet-Selch 

First off thank you. The problem with emet is that he was mostly a loner doing things on his own. He would occasionally order other ascians to do certain things, but usually he would just set event in motion himself. Morever, He state in the first cutscene that we meet him that Elidabus summoned him back to help with the next calamity + him be the core to Zodiark make him the most important of the paragons. Also Elidabus's mental decline only had to do with his since of self as he was slowly becoming a robot who's only purpose was the rejoining. Ultimately it boils down to Emet being to much of a loner and Lahabrea being to insane(technically going by the convocation he would be as he was the speaker) Now as for the Myth part Zeus isn't necessarily the strongest god. In fact Athena, his daughter, was supposed to overthrow him like he did to his father according to a prophecy he received; however, be a goddess of wisdom she decided against it. Zeus feared the Goddess of the night Nyx so much that he wouldn' t even attempt to wrong her, and when it comes to his brothers to my knowledge a distention between strength is never made in fact they're often shown as equals with all three of them needed to defeat the titans.

Depends on the myth. In some he kidnaps her, in some Zeus brings her to him, hell in one she goes to the underworld without hades knowing to escape her mother. In Demeter's story it's actually revealed that Hades first asked Zeus for her hand to which he agreed(Zeus was the one that didn't inform either of them)