There are two things you're addressing here... spinning plates and the idea that a woman prefers men who are capable of attracting women.
Before I entered a LTR, I had a policy of outright honesty with my plates. That does not mean I sat down with the girls I was dating/f*cking and explained to them in a detailed way what I was doing. At the end of the day, I think anytime you have a girl who is psychologically intact, she is going to develop an attachment to you if you are having a sexual relationship. There are some girls who will be spinning plates right along with you, but that suggests to me that their ability to pair bond has been compromised. There are also some girls who will continue to f*ck you but have some hypergamous lust for another man that is for some reason unavailable to her. Thus, you placate her desires.
Anyways, after a certain period of time with the sane plates I was spinning, they would start making veiled references to attachment and I deflected them or didn't address them. I'm sure you know how to do this and it's largely up to your own personal style.
There usually comes a time though where the woman isn't going to be satisfied with subtleties and will want to try and catch the "Great Catch". This is when they ask the question "where are we" and its many derivatives.
Go back to the principles you know - a woman would rather share a man with options than have sole position of a man who can't get women. Do you believe this or do you not? It sounds like you're worried about one of your girls finding out that she's not your only one and that she's going to derail the gravy train. Unless she's crazy I don't think that will happen. You have probably set a strong frame and she understands that if she did anything like that she would lose access to you. And if you have been the man that was able to obtain a collection of plates and engender a level of attraction like that in one of them, why would she be any different. At least, that's how it has seemed to work for me. I never had issues like that.
When the issue came up, I was always outright honest with the girls and told them I wasn't looking for a relationship... and I wasn't, with her. Don't be afraid to be dominant. You have been being subtle with your quasi no contact strategy and that's fine, but it's probably not going to give you the results you want. Guys analyze all these "signs" women give them with no intention of actually figuring out what's going on - they just want a rationalization for whatever outcome they're hoping for. Women do exactly the same thing. This is also a "tactic" guys will use occasionally when they want to ramp up interest.
It will probably deflect the problem for the moment but if she really is becoming attached, you'll have to address it head on later.
And I really don't think it will be an issue regarding your other plates. Again, a woman would rather share a man with options than have sole possession of a guy who can't get women. Do you believe that?