I think Rune's ending is a 'canon' one with game's throwing player out of control to the main menu (without mods).
I have a very rough idea how sexmaingirl mod works, but I have no idea how to fix that behavior.
As for required graphics, I can give you an idea how to make an NPC character to 'work' like slave:
View attachment 2779995
That's just a main portion of graphics with individual skeletons one have to deal with. There's also capsule, tentacles, gloryholes, horse. Code alone won't cut it, so like graphics alone without a proper code too. Oh, and we can't just put custom graphics for NPCs without rebuilding the project.
Time and resources for devs. Totally get it why some stuff had to be axed during development.
I honestly doubt it that modding will add that. I can only try to bring more clothes/hairs. Only existing in-game designs, mind you, because at this point I doubt anyone here will bother to draw custom sprites.