To start off with, I'm alright with the JSDF being the center of the story. When I think about it, if this was created in America, or Great Britain, or any other country, then the same thing would happen, in that the main country's army would be the hero, with everyone else being inept and useless. It's just something that is common in media about a nation's military being the heroes.
It also helps take the edge off considering all the adoring shoutouts to American military media there are, up to and including the Ride of the Valkyries scene. Not to mention that most of the JSDF's gear is second hand from the US.
That said, Japan is in a relatively unique political position in the world, at least when compared to most first-world countries. Sure, the idea of a nation surrounded by three aggressive yet nominally friendly ones is something to write about, the fact of the matter is, whatever your current feeling about the United States military, Japan really is supposed to let them take care of larger threats, with their own forces in a purely defensive role. And we got to be in this role through military conquest. A conquest, I might add, that we can very easily reinforce via the very thing that allowed it to come about...
Ok, so you want to make America one of the antagonists. Fine. It might come as a shock to some people, but many Americans
don't mind portraying Americans as the bad guy. Though we would prefer it if we were the cool badass bad guy.
And you want to use them to portray how Japan should break away from the treaty, ok. But play with it a little bit, don't just write them into a single scene where they get slaughtered, then forget them. Have them be active
antagonists, people who are supposed to be there to help, but are letting their "pushy brutish Imperialism" get in the way of that. Y'know the CIA? Have them do all the rotten things people think they do, like arming the Imperials with modern weapons. We don't mind, since we portray them as doing this anyway!
Personally, I would go down the "Worthy Rivals" route. Have a squad of US Special Forces, with a foil of Itami as the CO, be there as "advisors". Maybe the foil be an LT who just barely passed selection despite having to work three times harder than everyone else. Maybe even on top of that, have him be a sci-fi geek
who's not that into all the fantasy shenanigans going on around him. Maybe even play him as a tragic hero, who is so reviled with the entire world ("Untold light-years, through a gate of untold power, only to run into the SAME DAMN SHIT!") that he's willing to go to any lengths to tear everything down to build what he believes is humanity's future ("All the talk of colonizing other planets, when we have one right here for the taking."). Toss in some cultural posturing for flavor ("Yeah, you've been quite honorable so far. Good thing we beat seven shades of sense into ya all those years ago, eh?"). They could even add Rory's slaughtering of the SpecOps groups in the canon proper as an extra motivation to work against the heroes ("Those CIA jackasses are idiots, but you still took American lives, and sexually enjoyed it, you freak!
") And frankly, a squad of US Special Forces in Exo-Suits fighting Rory would make for an awesome fight scene, particularly if they play up how they can lift her axe while they have the suits on.
But yeah, there's a good reason there's at least two
fan fics that deal with a friendly US force assisting the JSDF.