I don't understand how the None Master works. If I create a document with no margin and no columns, then go to the A Master and add margin and columns, sometimes the None Master will delete the margins and columns, and sometimes it seems to adopt the margins and columns from the A Master.

I would recommend that the None Master ALWAYS delete all the columns and margins so it's a blank page. Or at least consistently return to the columns and margins of the A Master, but blank would be better.