For those who’ve gone through a divorce with a high-conflict person, how did you keep your resolve when they made last-minute gestures of working on themselves or tried to derail your plans?
If she claims to be working on herself, then that's fantastic...encourage her to keep up the good work. This is a person you love (or loved) and of course you want to see what's best for them, despite what's happened and the current situation. So be encouraging, even though you're not seeing progress.
However, her last minute attempts to reverse years of abuse doesn't mean you should just blindly accept things from her perspective. BPDs work in cycles, ranging from idolizing their partners to despising them. Who's to say this is any different, or that changes will be long-term?
Again, support her desire to receive therapy and make meaningful changes. That has nothing to do with whether or not you should file for divorce.
How do you navigate the period between emotionally detaching and filing when you’re still living together?
You do the best that you can every single day, and you get through it however you can. You focus on what's best for the kid and what's best for your own mental health, even though things are a mess at the moment.
If you're dead-set on divorce, then this is the only path forward and it requires quite a bit of steps. All you can do is take one step at a time and react appropriately when she challenges you. If that means leaving early, so be it. If it means being kind and patient, even when it feels like the last thing she deserves, then so be it. You just keep moving forward and do the best you can.