Got it resolved. As you suggested, I had messaged them on the original approved dossier to please allow me to revise it. I'm not really convinced that I would have gotten a response or not; I think the difference was my wife calling the offices there ... actually, spoke today to people in ETIS; two people in Haut Commissariat; Chief of staff at Ministère du Tourisme; and Tahiti Tourism in Los Angeles. Eventually, Haut Commissariat simply enabled the dossier to be edited, I went in and made the date change, and 10 minutes later got a new Travel Authorization letter. So, I think I'm good to go to travel tomorrow... finally.