Yes, Mon Mothma is a very minor character. Especially before the Andor show, which arguably gives her too much screentime anyway. General audiences wouldn’t freak out, they would say “who is she supposed to be?” You would have to develop her for a little while before you make it a big twist.
I’m not too familiar with the Yuuzhan Vong but I think that’s the kind of direction you have to go if you don’t want to repeat the Sith and the Empire again. Lucas’s idea of underworld criminal elements filling in the power vacuum left by the Empire is also good (though I’m iffy on it having Darth Maul.)
Being that this is Star Wars, I don’t believe there are ‘minor’ characters anymore. That’s why every scene in the Mandalorian has some kind of callback be it an alien, a piece of hardware etc. We even had the cantina guys repeat their '77 dialogue in Rogue One!
It would be so easy to generate a nostalgic connection to Mothma in ep 7. One early appearance in an RoTJ-esque outfit, with Leia saying “it’s so good to see you my friend - we’ve missed you terribly” would bring a rousing cheer and establish a ‘between films’ connection that would fuel the Ben Solo thread. Maybe he attended university in the Bothan system and stayed with the Mothmas?
The criminal underworld thing is plausible of course, but as much as we hate to admit it we do want the Sith and Empire again! Of course we do. Casual audiences definitely do. Again, the Mothma idea at least gives credence to the notion of a reconstituted status quo rather than the obvious ‘uh, the Empire grew back’ thing of Zhan’s novels, the Dark Horse comics, and the sequels. Again, it leaves the OT victory entirely intact whilst presenting the sinister idea that one of the lovely characters in the Rebel briefing room was actually a baddy with a future axe to grind…