Update: I replaced my WIU very quickly (<1 week) through USPS; thankfully I'd purchased through forum sponsor Legacy Station who were very understanding, supportive, and fast to replace the faulty unit.
Once in-hand, setting up the WIU only took a few minutes, maybe five minutes total including configuring it with my existing home router and getting the app on my iPhone. Really slick, and kids just loved it. Now that I have a functioning WIU in-hand, proper USB-serial adapter cable, and DCS 5.0-upgraded TIU and remote -- everything works great together. It's extremely responsive.
Thanks again to Barry for the troubleshooting help and Legacy Station for the great service and support.
Bob