I have a list of initial thoughts... longer than I intended.
Spouses with BPD have exceptionally excellent senses, more than just for manipulation and pressuring, cajoling, interrogating, etc. Likely she will sense that *something* has changed, may sense that you're contemplating a separation or divorce and therefore she may try to race you to filing a divorce case at the court house to claim you're Dr Evil Personified, demand the moon and put you in a defensive posture. It has happened to others.
One goal should be to seek the best - or "least bad" - temp order, both in your initial filing as well as in the initial court hearing where a temp order will be written. Why not let whatever happen and have the lawyer "fix" it later? After all, isn't it just a temporary "temp order"? Temp order can continue unchanged during the entire divorce case. (My story... My divorce took two years and though magistrate, lawyers and other connected officials knew there were problems with my lousy temp order, literally no one tried to change it, not until the final decree.) So think ahead and be proactive with your lawyer in anticipating likely obstructions and proposing solutions well before the court can blunder into decisions difficult to undo.
Focus on the more important issues. Securing authority in your son's guardianship will probably be top of your list. Same for your son's various needs and housing. Ponder how to address sticky scenarios. For example, if your son resides with you, then stbEx will want to remain in the home since then it will be more likely she will argue to be the one in charge of his care.
Oh, and don't be surprised if any/all your joint accounts are raided. You would would of course be fair and not withdraw or withhold more than your ethical 50% but pwBPD aren't prone to think in that honorable way. And after it happens the lawyers would just say "don't worry, we'll fix it when we get to the financial reconciliation at the end of the divorce a year or so down the road"... and then totally forget all about it in the rush to close the case on settlement day.
How the finances are divided, cars retitled, how the home is sold and equity split or residences separated, all those details often aren't finalized until closer to the end of the divorce. At first, court will focus on the initial basic living & support general framework, then deeper custody issues are wrapped up later. Last is splitting the assets and debts. Our lawyers rushed through the last part in what felt like 15 minutes.
This is not the time for our natural inclinations of fairness and being super-fair or super-sharing of information. Yes, never ever be nasty - behave as though the judge were looking over your shoulder 24/7 but otherwise be aware that your stbEx may be allowed to be outrageous with little or no consequences. Court will view it as bickering spouses and expect it to reduce after it is over. Court may not care how mentally disturbed either of you are. (My court studiously ignored all my ex's antics, probably with that philosophy.) You won't get credit for behaving well and your spouse won't get much in the way of consequences for behaving poorly. Sad but generally true.


