Nice work, Barbara!
Some people have questioned the possibility of Jack having an eight-month-old baby by the time of the reunion, and it does seem unlikely.
But the part of the timeline I have the most trouble with is the end. How is it possible that Ennis has been dating Cassie for four years and yet so casually mentions her for the first time on their last night? Either he would have casually mentioned her years before, or he would keep her a secret entirely. It's just hard to imagine him saying, "I've been putting the blocks to a good-looking gal in Riverton for the past four years."
Jack has presumably been seeing Randall for about as long (we don't know that they got together immediately after the charity dance, but I can't imagine they would have waited years). Though he actually IS keeping Randall a secret, so the same argument doesn't quite apply.