Hi scobberlotcher ,
It sounds like you are approaching things from a really good place. Boundaries really are the key to keeping things in line. Many of us nons have difficulty in boundaries. It's important to remember that when setting boundaries, you are not putting limits on the other person. That is just control. Instead you are setting boundaries around what you will and will not tolerate, such as how you are treated. For instance, when my H begins to yell at me, I will tell him that I don't like to be yelled at and I'm going to go take a walk until things calm down. He can continue to yell all he wants, I'm just won't be there to listen to it. THere's a REALLY good book called Boundaries by Dr. Henry Cloud. It has helped me understand how to figure out what boundaries to set and what isn't a boundary. Of course, as a couple, the two of you can set boundaries together, such as with the spending. My H and I also get an allowance every 2 weeks. I can make mine last, save it back for larger items, but my H is incapable for saving his. He quickly spends his every week.
The best thing to do when you pwBPD is dysregulating is to validate. LIsten. But don't fix it. It's imortant that they learn their own coping mechanisms when they are having problems. For my H I ask him questions to try to lead him to the correct response. Sometimes it works, sometimes it doesn't. WE have a lot of workshops listed on the right side of the page that can help you navigate through a relationship with a pwBPD. To start with, here is one of our workshops that may help you determine how to support and not enable:
https://bpdfamily.com/message_board/index.php?topic=95263.0