The whole historic timeline isnt that much... about what? 700 years? so the spaceship (call it the arc) crashed on on this world inhabited by tribal creatures. From this Arc are the several playable classes that and groups, which forged each independet nations (maybe it was first several task forces to explore this new world). With time they lost the knowledge to use all the stuff you would now labeling "magic" (magitec anyone?). So.... lets assume that this seventh umbral(?) is a kind of event happening every 100 years (crashed reactor chain reaction consuming the materia around it; and like forging on an ambos; creating the surroundings, and possible alternative worlds anew (maybe explains the echo-power). If you see all the previous storys of Final Fantasy everything is possible.

About the Primals.... what if ALL creatures on this world slowly learning to use magictec (crystals etc) in some way or another. If you did the Ifrit Pre-Quest you have been told that summoning the primals is slowly consuming the worlds energy (Hello FF7 Shinra Corp!). So... lets assume that the Garleans are pissed and want to take off of this planet asap (more advanced technologie) most of the people want to stay on this world (may it be they simply like it, don't know anything else OR some of the twelve know what happend in the passed -> maybe old world exploded) and the primals don't know what they are doing (summoning = killung the planet). It would be like giving naive childs some nuklear warheads to play....

Here we are.... more or less 3 Factions fighting for different views of the future... who will win? We don't know....