I know you've moved on to the specifics but I wanted to chime in on this specific question.
Do you ever feel like you need help with a plot point in your book, but don't want to ask for help because it might lead to issues later? Like someone wanting money for their idea that helped you. I need some help with something I think is shit. Normally, I would just tough it out until I could figure something out, but I've already been working on this one book for too long.
I've never had a problem asking friends, fellow writers, family, or chatty strangers to weigh in on something I'm not happy with as long the discussion is broad enough. I also have a circle of fairly involved beta readers who are usually pretty happy to suggest changes including alternate wording and specific plot points that may or may not be used in the editing process. Everything is tracked through comment logging and there's really no room for anyone to claim credit for anything.
HOWEVER, I also run Roleplaying games in my meta-setting and one thing that is made abundantly clear to every player is that while their activities my intersect with larger plot lines, none of their characters will EVER appear in any published work. It's just not worth the headache of someone potentially trying to claim credit for creation of a character.