I think the only way to truly handle this situation is to develop an edit history. This would allow users full, unlimited editing of all their posts all the time. Robin could place a button to click near the bottom right corner of the post that says "post edited" and when you click on that button it would show all the previous edits, from most current to oldest, as well as the date and time each version of the post was originally posted. This would allow the post to look clean and not filled with edit junk, but that the original versions could still be uncovered by anyone.
Say someone edits out their hateful comments, leaving a one-sided argument. You'd be able to click on the Edit History button on the post and see all of the previous edits.