Mario, Link and Star Fox have all had sexuality at one point or another and plenty of side characters in other series have sexuality too. It's not just Fire Emblem.
On that point, is it safe to assume Three Houses will never release in China?
I think that whatever amount of sexuality Mario, link, or Star Fox has had in the past, if it had the equivalent in homosexuality it would get past the censors. Like if mario rescued toad instead of princess toadstool I think China wouldn't give a fuck. Seriously I haven't played every mario, Zelda or Star Fox but I can't remember any sexual situations in any of them.
The new Fire Emblem I haven't played yet, so I don't know how gay it gets, so to speak, but it's already in Simplified Chinese so it shouldn't be too hard for them to apply to sell it in China. It's easily available online and at game retailers already (same as every other switch related thing). I have no guess either way on if it would be approved or not if they tried.
Honestly I don't know the exact rules for portraying LGBTQ+ people in Chinese media, I might be wrong but I don't think there is an outright ban on homosexuality. There are plenty of "flamboyant" or "feminine" guys on tv that may or may not be gay (I know it's not really good to assume that they are gay but I don't really pay attention when my gf is watching chinese tv), and there is at least one pretty famous transgender star that's always on tv. You don't see guys kissing or anything like that though.
I'd guess if the homosexuality in FE isn't overt then it's fine. Or Nintendo could always localize the game for the Chinese audience and censors if they thought it was worth their time and money.