Hm... . I am really sorry that the communication between you and at least the ex's parents isn't possible. It would make the situation easier to manage... .
Trying to figure out some boundaries and consequences when it also involves your grandson is much harder than when it only involves you and the pwBPD, because if there are negative reactions, it will affect everyone in the mix. So, that is something to consider as well.
The basics remain the same though:
First, you need to really figure out what you are willing to do and what you are not willing to do in this situation. Then you can devise a strategy as to how to achieve that. Are you ok with driving all the way? If you absolutely do not want to do that, you can enforce boundaries.
Are you afraid that she will not let your grandson stay again if you don't give in to her? If so, she probably knows that and is using it to blackmail you. Is that something you feel strong enough about to call her bluff? What if she isn't bluffing - is it likely you will lose contact with your grandson? If so, is it worth it?
In the example that you give:
What is happening? You know she has nobody to drive him, so you drive him all the way? Or you come to the half-way point and then find out it's up to you to drive him all the way?
If it's the first scenario, and she called to tell me that she doesn't have anyone to drive him, I'd say: "Ok, honey. Let me know when you can pick him up, and we'll drive him to the half-way point." If she pressed me to drive him all the way, I'd say: "I can drive him to the half-way point. That was the original agreement. I am sorry, I will not be able to drive him all the way."
If it's the second scenario, I would come, call her and leave a message or text: "Hi honey, we are here at the half-way point, we are ready to drop off your son. Let us know what's happening." I would wait 15 min, half an hour at the most, doing something fun with the gs, and if she did not contact me, I'd go back home and let her pick him up at my place.
She would most likely be very upset - especially if her son would miss school the next day. If she knew this was non-negotiable, she would get the picture after a few mishaps.
In the meantime, you would need to:
1. Be willing to not see your grandson, if it's an issue that is important enough to you.
2. Stay strong and not give in (if you gave in, you would be giving her what's called
"intermittent reinforcement" and would be worse off then before)
3. Validate her feelings about her difficulties, stay calm and positive, yet firm about your boundary.
Depending on your age, health, time-constraints: do you think this would be something worthwhile in your situation?