"Any development team would rather make one version of the game that would work for all regions"
That's exactly where I will agree on this one part, because almost every first party switch game ever has a universal SKU that works across all regions. This game included. The only thing that alters between languages are the text and sometimes in-game artwork if the company goes that far (Yugioh on Switch will use the Japanese card art if you play in Japanese) and games are thus made with a "Worldwide" mindset in mind so that they don't have to make a billion versions per country.
The thing that's been done is that TMS has been altered for the worldwide view due to the fact that some of the US edits have been backported into other parts of the FE series such as some merch (Kiria's US uniform was put on a FE Cipher card) so it basically became the new canon in the same way that the extra Weapon battles in FF7 were put in newer FF7 ports in Japan, despite them being US additions. So clearly, the developers are fine with using it on a worldwide scale to release it in Japan with the same Cero B rating.
"But not really the western world... a small fraction of the western world that visits Resetera and complains on Twitter about games they would never buy no matter what content was in them."
First off, it's kinda ironic that you single out a random forum when I've never seen people there outright beg for the original game to make the edits that they did. Most people there poke fun of the dumb wedding dress outfit that's just like a bad paint bucket edit.
If anything, i've seen more people on the internet bitch about no vagina bones and the paint edits taking place, than people actually begging for them covered up/altered, which basically just shows NOA/Atlus/the staff did the edits on their own due to their own assumptions about the western market without being told to by anyone. That's how companies do most edits in fact (that or the ESRB tells them outright).
It wasn't secret outrage that made them do it out of fear, they just... did it. Just like they do for other games behind-the-scenes during betas that nobody outside of the dev team is aware of. I wouldn't be surprised if during development for games like Xenoblade 2 or Pokemon, a dev member thinks of something and then someone else goes "wait, maybe you shouldn't do that because ___ may not like it or ____ will work better", so it's changed. It's no different than the rare times where you see a tone shift in a video game from a pre-release trailer to the full game.
Also not to mention that aging up characters is literally a constant common sense edit for any sort of localization even if everything else about a game is unchanged. I've seen some M rated games that do this with no other edits because it's better to age up and be safe than not age up and come off as walking a dangerous tightrope, and aging up is usually the only thing I've seen people occasionally want when it comes to a game being brought over. The Bravely games had it, this had it, Xenoblade X had it, it's common.
Really the only thing i'm worried about here is the hot springs DLC getting ignored again since it was DLC content that shouldn't really go to waste and I thought it was dumb that instead of editing it in the Wii u version they just... Didn't release it, but besides that I just want a fun game and a great story to enjoy... And I'm still getting that since I enjoyed the four chapters I played of the Wii U version and I want to finally 100% it.