You can be in sub path as well and trigger the NTR path, not only on dom pimp. As you've discovered, the triggers are Sue upset with you and Sue has sex with Theo after watching you with Scarlet (without Lucia) > then the NTR path is up and unlocks the last scene. I think the NTR path and the last scene triggering will render the SuePimp variable to false.
Dominant pimp is not a blanket variable for all LIs, you need to establish it for each, at least this is how I think it works and seen it work. It is true that Sue or Lucia (or both) might fool around with Theo on that first night without MCs knowledge/permission, while on the pimp route. I found that to be a bit odd too. I think that will come to light and be addressed in a future update, with potential to keep the route or not, depending on the choice that MC will take when he finds out.
As far as I've seen, the MeganPimp variable is not used yet. I have some saves with both scenes in Megan's path and the variable is unknown for both. Maybe it will be used later, dunno.