So out of total frustration and just refusing to give up (after 40 years in IT,) I bought a literal junker IBM Thinkpad T20 with one port. No NIC, no USB, just one serial port. Upgraded the OS to XP SP2 and .Net 2.5. That took 4 hours.
Then I downloaded the new DCS Loader and all the 4.2 software from MTH.
Tonight I took it down to the trainroom and WHAM! Upgraded two TIUs and the other remote in less than 15 minutes!
Now we're flying with 2 remotes and 2 TIUs in Supermode, all running the latest 4.2!
Just needed to "get back to basics" and give up on the new computers and their fancy-dancey operating systems.
Good ole XP and T20 pull it off!
A very good couple of days.
Just goes prove Galaxy Quest was right: "Never Give Up; Never Surrender!"