We've had a Guardian ad Litem suggest she can only have him up to 8 hours, we've tried mediation, and many other negotiations to avoid a very expensive and lengthy trial, but she does not see herself the way others do.
The BPD mom usually see herself as the best parent. Doesn't matter how much you argue the opposite, the BPD won't understand it. They live in what seems a different reality.
Not sure how it works where you are but if the state pays for the GAL, I'd let the GAL fight that battle. Negotiations are unlikely to lead anywhere. Yes, you'll have to pay for your lawyer and things will take forever, but don't hope too much from mediation.
Be careful about pushing for too many visits. Every time the child goes to visit mom, they'll likely be stressed the day before and stressed the day after and you'll have to deal with this. Not fun and I speak from experience. You can't fix the mom, and yes, while mom should play a role in their life, don't try to force that. Kids need stability first. A BPD parent is completely the opposite.