Hi! I relate to how confusing it can be to try to understand the logic and reasoning behind a person with BPD's actions.
I am not an expert, and so instead of directly answering your questions I recommend that you check out the ressources available on this website (
https://bpdfamily.com/message_board/index.php?topic=347836.0). You start learning that there are actual terms that describe all these different behaviours, like splitting, devaluing, clivage, as well as exploring your part to play in the relationship dynamic. These readings have certainly been helping me a lot!