What's the emergency here? You have stability in the town you've moved to. You have a great job and are settled into a house that will accommodate her when she is ready to be there. Presumably she is not the only good thing about this place for you.
Whatever she's going through, do you not have the time to gently communicate and process with her at the slower pace she seems to need right now?
You ask "Do we walk away?" and "How long do we deal with this before we say 'enough'?" -- but have you asked her how she's doing, and what she's thinking these days about those plans to move in together (assuming you all had articulated such plans before you moved)? You've loved her for 18 years. Do you not feel comfortable after all that time saying something like, "I was hoping you'd be moved in by now. I miss some of the intimacy we'd built over the past few years. What are you thinking these days about where we are and where we're going?"
Are you only willing to continue the relationship with her if it is exactly what you expected it would be? Do you feel the need to be "free" of her if she's not going to move in and "be a triad" with you (whatever that means to all of you)?
It sounds like you are feeling a lot of pressure for her to fit a specific slot. Maybe that's translating to her feeling a lot of pressure to very specifically fill that role. Even if she agreed to that, it might feel pretty intimidating to face the test of it, especially if she's having second thoughts about certain aspects of it, or just generally feeling that it wouldn't allow her or your relationships room to change.
It sounds like some loving communication is in order. And not the kind that sounds like "we might just walk away" or " how long are we supposed to take this?" Maybe more the kind that sounds like "What do you need right now?" and "What aspects of our lives do we (each and all) want to share, now that we're in the same town?"
Demands that she fulfill promises are probably not the kind of loving support she needs (no matter how righteous you feel from the position of having fulfilled all of yours). Openness to her feelings, concerns, and ideas would serve you all well.
