yeahbut. Sometimes person B may reply to something said by person A but attach it to the thread by linking to the last item, which could be by person x, y, or z...

Surely if you set your display preference to 99 posts per page, there is no quicker way to view everything, and above all get the overall context of an unfolding (or collapsing) discussion? When the thread opens, you just hit the <end> button, go back over the last couple of posts (or more if required for context or previous comments), and when you are done with that thread, just hit the Next Thread / Previous Thread buttons, for a one-stop move.