So, I thought about ways to reconcile the fact that Fates' occasional narrative inconsistencies and ridiculous villains with the fact that it canonically coexists with Awakening's generally coherent narrative (albeit, Awakening has its own bits of silliness all over--Grima and Validar's status as generic doomsday villains, mostly, but that's beside the point.)
I considered the Before Awakening DLC, in which Chrom mentions that "everybody knows about the mythical kingdoms of Hoshido and Nohr". Despite the fact that Before Awakening is obviously non-canon and is more or less
continuity fanservice, that got me thinking: what if Fates is actually just a legend concocted in Akaneia? The base of the story would've been something that'd evolved over the years from the same ideas, but cultural divisions led to the route differences--each version of Fates is one culture's version of the story.
Birthright is the story as told by Ylisse/(The general cluster of countries Ylisse was formed from). Conquest is as told by Plegia/(The general cluster of FE1/3 "bad guy" countries Plegia was formed from). Revelation is the story as told by Regna Ferox.
Garon would be symbolic of the apparent parade of bad leaders Plegia ended up with--hence his almost comical, puppy-kicking evilness; he's a sort of pastiche of bad rulers and warmongers in general. To the Ylisseans and Feroxi, this was representative of the threat that Plegia seemed to pose, while to the Plegians, this was a manifestation of a sort of want for internal reform, immortalized in culture, and Nohrrin's decision to side with them is their hope for a leader who can get Plegia out of the hard times it's clearly had prior to Awakening.
Mikoto, meanwhile, is representative of the opposite--Ylisse's heroic lineage of Anri and Marth was immortalized in her character. Under the ideas of the Ylisseans, she's representative of their best leaders combined. When the story was given the Plegian spin, she took on the additional notion of Plegia's jealousy of Ylisse and want for a better leader and better country in general. The Feroxi story takes a sort of middle ground and acknowledges the vital role that Ylisse's founders played in the continent's past (cough, cough, FE1) but that their individuality as a country is important. Birthright!Corrin deciding to side with Ylisse isn't so much a symbolic element as a moral example for children.
[
Anankos, meanwhile, is a sort of mangled take on Medeus, where the only remembered elements were that A) he was a dragon and B) he was a dick, and this poorly-remembered, antagonistic historical figure eventually got sort of rebuilt into the sort of tragic character Anankos is revealed to be in Hidden Truths. Revelation in general is an allegory for the true ending of FE3, with R!Corrin discovering that the conflict was orchestrated by a totally different and seemingly forgotten enemy (akin to collecting all the MacGuffins in Book 2 of FE3 lets you fight Medeus as the true final boss rather than Hardin.) Their courage in taking a third option and the overall strength they display in defeating Anankos are reflections of Feroxi morals.]
The actual events of Hidden Truths and the presence of Odin, Laslow and Selena is... uh... something I haven't thought out entirely yet. My best idea so far is some sort of extended Einherjar (or however it's spelled) effect--some sort of spacetime anomaly created an alternate dimension where the myth became fact.
Alternatively, the theory can be shifted to Valm, with Chon'sin taking the place of Ylisse, Valm taking the place of Plegia, Roseanne being switched with Regna Ferox, and Doma/Duma from Gaiden in Medeus's place--the obvious Japan-ness of Hoshido supports that version more, but we aren't given a very clear view of the political climate on Valm and it's quite possible they don't transplant quite as cleanly.
All of this probably sounds silly and overanalytical, but I'd put some thought into it and was curious what you all thought. :v