I think it is possible have your feelings and needs heard, but often you have to get theirs feelings out of the way first. They view the world through rejection and when we approach them about something they did wrong by accusing them they will respond out of rejection first, anger second. It is hard for them to admit when they do wrong because that means they are "bad", which means they will be rejected.
One way to say hard things to to someone with BPD is to use
SET . We give a supporting statement, an empathetic statement, and then the truth.
Using your example of her being late:
You: I think it's great that you hang out with X every now and then. It's important to me that you have friends and have a good time with others. It's hurtful to me you're going to be late and I don't hear from you. Can you please just let me know that you will be later than planned next time?
Using the SET model and your 2nd example, would you practice a way to address the pool part here?