Hi morgan. It is good to see you here as this is a place where lots of people can support and understand.
I am not sure how you can stop her from moving to your city if she is determined to move there. Short of a miracle that is, so keep praying! It may be best for now to focus on how you can maintain distance from her should she move there. I imagine you would not want her staying with you so make that clear to her. If it were me, I would not even let her stay a night or two as that can be extended and then extended and become quite the mess. The other thing is for you to start working on deciding what your boundaries are and how you will maintain them. Boundaries will keep you safe and can help you stay clear headed during interactions with your mother.
Reading your post I was struck by just how supportive and caring you have been. I admire that but I can also hear that you are tired of her behavior. Would you feel comfortable telling her that while you do care, you can not really trust her to not pull her disappearing act after the way she has done that very thing time after time? Do you want a "christian mother" when you are almost 30? It sounds like you are more than capable of taking care of yourself... .how would you feel telling her that? (Forgive me, while I was raised Christian, I have no idea what a Christian mother is and all I can envision is a smothering interfering, abuse-me-with-biblical-quotes woman chasing me around so my comments may be way off here. No offense is intended).
I've decided I do NOT want her here and now she is hell-bent on moving here. She's actually already updated her LinkedIn to say she lives here! I'm so terrified of what mess is upon my life. I've worked so hard as a young adult to GET AWAY from MO and all the drama she created there. And now she's determined to move here and is completely delusional about thinking "God wants her to".
Morgan, if I were in your position I would be nervous and worried too. But it is doable.

If she does move there, there are ways to communicate and interact with her where you maintain your own life and minimize any damage or interference from her. Think of this as an opportunity to erect and enforce boundaries (which are always good things to have) and to learn how to temper what seems to be a natural (and wonderful) sense of compassion, patience and forgiveness. Those are wonderful qualities, but sometimes they can work against us when dealing with family members with BPD.
Did your counselor have any constructive advice for how to handle things? Suggestions for how to get her to not move (I can't think of any)? Or offer to help you work things through in terms of how you can do some of the things I mentioned above?
Keep posting morgan. We can support you as you strategize and figure things out at the very least.