I am accused of being selfish and sticking my unwanted opinion into the situation. I am told that I’m a narcissist for having something to share in the conversation. I apparently “interrupt and hog the conversation”when she’s talking.
I think I can check those on my list too

Do you want to keep putting up with that? If not, try setting boundaries to stop any verbal abuse. If it continues after a warning, simply say you need some alone time and exit the room. It sounds like you've become her caretaker and she expects an unbalanced relationship. I recommend reading Stop Caretaking the Borderline or Narcissist as inspiration to change the paradigm in your relationship to become more healthy. It can be hard in the short term but with long term benefits.
Do they just not want to hear our ideas? Do we need to preface with “could I say something?” Or “this could be another way to think about it”
Maybe. They can take someone having a different opinion as an attack even if it's not meant to be. Prefacing can help -- see what works for you. I'd echo the boundaries comment above. I think it's important to have a support network of friends and family (other than your BPD loved one) so you can have more normal interactions and ground yourself.