I understand how important the concept of no contact is, but I don't think it is totally attainable for everyone,
You are correct - it is not reasonable for everyone. NC is a tool to aid the detachment process - when feasible can help tremendously getting out of the FOG.
Going through the legal process of separating and divorce, it seems some contact is actually necessary, is it an indicator of personal progress that I can chose when to communicate with BPD and when I can politely hang up after a warning?
I am not sure I understand what you mean in terms of communication and hanging up. My ex and I had LC (limited contact) through the divorce time. We had to deal with taxes and such and managed to do it rather cleanly although it took a lot of effort - I used staying board communication tools during this time. For the most part, everything legal - the attorneys handled.
It was important for my own healing for very good boundaries.