Counseling imo only works with full effectiveness with full honesty. Living in a world of secrets isn't a world that's relaxed, walking on egg shells comes to mind.
I have a mother that I highly suspect has BPD and is in full denial. It's hard for us as we cannot get confirmation to satisfy us and help her.
Self harm or threats of same is to be taken very seriously however, it isn't for you to carry the full burden- she needs professional assistance whether she agrees with it or not. I'd be very direct with her on this e.g. if she self harms call an ambulance, if she threatens to but doesn't go ahead, attend your GP the following day...with her or without her on the basis that your mental well being is as important as hers.
Being a carer is a great role to help and support someone but you are not a miracle worker.
As for taking blame... take it only when you know you are in error, until then if you have doubts then It's ok to say "I don't know" or "I don't think so". It's ok not to know all the answers or have doubts or disagree, it's the other person's problem if such freedoms is not tolerated.
I hope that helps. Repost anytime.