This occurred after the Baltic Powers DLC. Game Version V.100.19.18309.0
Before the update, entering the scenario editor and placing a unit or building as any player other than 1 would allow you to control that unit as long as you were still placing stuff as that player without having to launch a playtest scenario. This was incredibly useful for testing but now it doesn’t seem to be possible anymore.
For the second part, it happens when using the Age of the World mod. Clicking on any hero customization button, even the entire vanilla roster, doesn’t do anything and makes it impossible to customize explorers unless the mod is disabled. Unsure whether other mods have this problem as well, but AotW doesn’t mess with vanilla homecity files at all.