I put it all here, it shows as a paste... mind looking and maybe editing and giving me your link? :)
Very difficult to come up with a solution without having test data.
If you were making forums, how would you order it?
Post date of the reply. Most likely by joining the last reply with the posts, defaulting the reply date to the post date if there are no replies yet.
Better if you paste the code to the article - more convenient for all.