OK, I have it working.
I believe the missing piece was, first, I ran the TWOS installer as administrator. That might have been part of it, but also, the PDF of the installation instructions here:
https://www.subsim.com/radioroom/sho...d.php?t=236598
does not match the installation instructions included with the modpack.
Critically, it does not tell you that you have to replace the Uplay/Steam sh5.exe with the DVD version (link to it on mediafire is included in the modpack installation instructions).
I did that, and now it seems to work. The game ran, I was able to save a game, and now I'm going to go open it and see if I can play.
Steve