Actually, you already can change allegiances between Grand Companies - you just need to be at Second Lieutenant rank before you're allowed to do so.
As for the thing about the Scions 'distancing themselves', that's just because
Minfilia got sick of various factions trying to manipulate her organization for their own ends after the Alliance's victory over Gaius and the XIVth Legion, particularly the members of the Syndicate in Ul'dah, and she wanted the Scions to be truly neutral in all affairs. Because Mor Dhona is centrally located in the middle of Aldenard and is regarded as 'no-man's land' so to speak unclaimed by any of the city states, it was the perfect place to relocate to. That doesn't mean they're turning their backs on the citystates however, just that Minfilia feels they're better off being truly independent.
As for being able to play as a Garlean, I seriously doubt that will ever be possible - the Garleans are constantly stated as being antagonists in the lore, and after the main scenario is completed, their power base in Eorzea is broken so they lost a lot of their clout anyway. Not to mention the internal power-plays stated as taking place in Garlemald itself - quite frankly it will be a miracle if the whole country doesn't collapse into civil war.
And besides, this is a Final Fantasy game - rarely, if ever, do you play as an 'antagonist'. This isn't like TOR or WoW where you have two opposing factions fighting each other. In Eorzea, we're all on the same side really, so if you're hoping to be able to play 'the opposition' you'll be sadly disappointed.