Goodness me. Quite a bit going on here! I've gone through and pulled out a few choice quotes that I feel I can help you address. Obviously, I only know as much as you've posted here so take it all with a pinch-of-salt.
"In the past, N has said she understands poly, but doesn't need it in her life. She has even said I could go have sex with anyone I wanted because she is not jealous in that way"
"Basically, rope bondage, flogging and all with a person she has an emotional connection with. This doesn't sound like poly at all to me."
First thing is that poly is about neither of these items. Polyamory is the development of strong, emotional and loving relationships. It's not about sex, that's swinging and it's not about bondage as that's BDSM. From what you've said, the rope bondage is much more poly than the having sex with whoever you want.
"These both happened with him knowing full well that my conversations with N are nowhere near a point where it feels ok for someone else to ask her such questions. It feels like disrespect for me, for N and for our relationship. Am I overreacting?"
If it's deception and not something your comfortable with, it's not poly. Another important aspect is full and complete disclosure of this kind of thing. He may have only said it as "this is an option" but N should be talking to you about this before she gives an answer.
"I press her for details (do you want to date, hold hands, kiss, have a boyfriend), but she just gets mad at me every time I ask for specifics."
There's a very good chance that she doesn't know. If you're both wired to monogamy then it's not surprising she doesn't know what she wants. This will come with time and experience.
"but I am worried about it crossing into poly."
You don't strike me as being 100% confident as poly and as a result my suggestion is to talk to N about this being a bad time for her to be wanting to explore polyamory.
Your relationship sounds very strained right now, and trust me adding more people will not fix anything. You and N need to come together and sort out your own relationship before getting involved with other people.
When you are in a good space, then N can explore getting into a D/s relationship with this other guy.