I don't know about question 2, but I do know that in order to disable putting strugglers into other partys and castles, they have been checked as "not moveable in the party window", which most likely includes the prisoner trade as well.
You can manually change that via the troop editor (ensure that you have a backup of the original file, and save the ID from the original lines, because the troop editor sometimes switches IDs) but you have to start a new game for that change to take effect.