I wanted to suggest house share but don’t know if that would be a good move for my own healing given the trauma of what I have witnessed.
Unless you're independently wealthy and can afford three homes (yours, ex's and a child's) then such an idea is not financially practical. Also, the child would not gain the knowledge that father's home and parenting was different from mother's home and parenting.
Thus the standard is that the children travel between each parent's home according to the court ordered schedule. In your daughter's case, she would not be transitioning homes since step-parenting usually doesn't continue post-marriage.
Be aware that many people with acting-out BPD traits (pwBPD) are too entitled, at least at first, to agree or be reasonable in mediation.
Even if you can't agree, there is no assurance a court ordered mediation will succeed. It all hinges on whether the other will set aside entitlement and self-oriented perceptions to be reasonable. You don't have to give in to the other's demands. It is okay to declare mediation failed and let the court process proceed.
That said, it is surprising that most of our divorces do end in a last minute settlement just after a major hearing or trial... after a year or two. Many conclude that judges (1) are deluged by the number of cases and (2) are reluctant to issue decisions. In my own separation and divorce I had at least a handful of continuances during those two years.
