The “all the bad guys- and only the bad guys- have red lightsabers” is a prequelism I can do without.
It’s not a prequelism.
Yes, it is. Or are there other darksiders in the OT with red lightsabers aside from Vader I’m unaware of?
The reason Darth had a red saber in Star Wars was to create the simple contrast of good and evil, just like everything else in the film.
That’s an out of universe reason. There’s no reason to think it’s an in-universe reason going off the OT alone.