Thank you both for responding!
I have had the sense from early on that D might fit the profile of 'cowgirl', and I expressed my concerns to K around that. His opinion was that whether she intended to break he and I up or not, we are the ones that ultimately control that, and that she'd eventually figure out that he's not going to leave me for her, and then she'd have to make her decision wheter to get used to poly or leave because it doesn't work for her. Forever the optimist, K keeps hoping she'll make a breakthrough. He does see that her derision about me around my condition is totally inappropriate, and defends me vigorously when she says anything directly. The difficult thing is that she's given to little sideways barbs, which K also calls her on. If K's support wasn't there, I'd be gone already.
I was also feeling hopeful the past few months that she was owning up to poly not working for her as she was talking about doing a campground hostess stint way from here, and so she would be essentially taking a break to figure out how she felt being alone (which she never did after leaving her ex). She was also thinking of going south for the winter in her RV. But she recently announced that she really still wants to live with K and isn't going anywhere without him. So we're back to square one.
Thank you, Nycindie, for the links!! I will definitely take a look!
I'm really glad I started posting. I have been following this forum long enough to know what a great bunch of people you are!!