The mechanic where you could change day/night was by entering the mirror world, yeah. Since the original Sun and Moon dex said Solgaleo and Lunala were male and female counterparts, it's been my head canon that in the OG games Nebby had a child with the Legend in that world.
On that note, if you have to lock this world's Nebby to one of the two Evos, I think Lunala would make sense. That way, the otherwise male Solgaleo from another world could be under the same R63 rules as Team Rainbow Rocket.