Hello there,
1. Sam is meant to be that way, she has solo content and share content, there's no slut route for her.
2. One more character will join the basement at most after a quest, but no others.
3. This is incorrect. While some people like to give it a "real life" explanation, there's none.
The term originates from old Japan to describe hermaphrodite body types, but in actual times is really mostly a genre for hentai and porn. Hence why futa comics/images and works can have futas who do not have a vagina. In short, by "real life terms" of when the term was created, you would be correct... but this is a work of fiction, they were "born" like that.