There's another angle here, I think. We're primarily looking at this from the "Poly people discussing Kink" angle.. it's important to remember that Kink is much more than just sex, too.
I'm not sure N is clear on that, maybe she is, but I feel like she's confused and thinking that Kink and Poly both are inextricably tied to sex.. which we know they're not... BUT... they often are combined, aren't they?
From the Kink point of view, most certainly there can be play without sexual contact... however, it's not as common for play to be done with the subject clothed.. it CAN, certainly, but there's a vulnerability that goes with being only partially clad that lends itself to the individual dropping into subspace. Nudity is not required, though... let's be clear.
But there's a lot of psycho-sexual energy flying around, that's true. It's a very sensual experience for a lot of us.
The other thing that there needs to be a lot of in a Kink relationship is TRUST... sound familiar? Yes.. there's an emotional and psychological bond that develops between a Top and a bottom, as well.. and this is where I certainly see the OP being concerned about it "developing" into Polyamory.
The bond between Top and bottom is a lot like love... it's trust mixed with dependence mixed with selflessness... it's very much like romance. It sometimes comes from or develops into romantic love. That definitely is something to consider.
There are a lot of feelings floating around, there's a lot of confusion as well... can I offer a thought? Have you considered a Lifestyle (both Kink and Poly) friendly counselor? Someone who won't focus on the alternative lifestyles, but rather on how these things are affecting your relationship.
I feel like you really do care for this woman... show her that you want to ensure that the choices you make are healthy for you both. If she won't talk directly to you, then perhaps an intermediary will help?
Best of luck to you both.