Don't do it. She's seeing someone else, yes? It took September through the beginning of February to get my ex to leave... while she was seeing her beau. It was an emotional hell for me. She phoned it in as a mother. She slept on the couch. Meals were largely separate (because I couldn't stand her face). She 9nce stayed out until 5AM while I was home with our kids. A teenage mom, basically. I didn't charge her rent or utilities so she could save up. We turned in the $40k SUV (2013 dollars) I had bought her at a loss to me of about $8k.
She wants something. Her past behaviors have demonstrated that you're not safe (she's not a safe person to be around). For pwBPD, "lying feels like survival." See
this post from a recent thread on Detaching. I read that book.
It also isn't her choice whether or not to sign a parenting plan. She could be served. I'd keep doing to soft approach though for now. I convinced my ex, the talks and negotiations starting when she lived with me, and ultimately served her by mail, no public shaming from a process server.
My ex also asked to come back (by text!) about four years after her marriage to the guy she left me for imploded. None of that was for me, but her.
Edit: my ex and I weren't married and I bought the house under my name.