OK, I figured out what was preventing me from successfully connecting to SatPC32 from Commander. I'm documenting my findings here in case it helps some poor soul in the future.
Note: This issue will NOT affect users who use only one account for everything in Windows 10 (likely the majority of hams). It manifested because my (perfectly legitimate) user account strategy collided with some assumptions on the part of the author of SatPC32.
SYMPTOM: In the SatPC32 panel of Commander, Clicking the Connect button had no effect. Status remained "Disconnected".
Commander and SatPC32 running at different privilege levels - Commander as a normal user, and SatPC32 as an administrator.
Why? Because SatPC32 would not start unless I used the 'Run as administrator' option in the right-click context menu. Attempts to run the program without it yielded the error "runtime error 217 at 00673791".
Why? Because SatPC32 could not find its configuration files.
Why? Because the installer placed the config files in the AppData folder of the Administrator user account, which the application could not see if not running as administrator.
Why? I have two accounts on this PC - let's call them User1 and User2. User1 is an administrator; User2 is a standard user. I always log in as User2, and elevate privileges using User1 only as necessary. When installing applications, Windows prompts me for the password for User1. However, the SatPC32 installer does not follow that convention, and requires one to run it as an administrator (install fails otherwise).
Following their instructions, I ran the installer using the 'Run as administrator' context menu, and provided the password for User1. This causes the installer to install in the context of User1.
It involves copying the SatPC32 folder of config files from the AppData path of User1 to that of User2. Now User2 can run the application as User2 without elevated rights. Now Commander is able to connect to SatPC32 and obtain Sat name, mode, and uplink/downlink frequency, so DxKeeper can grab that when logging a QSO.
Thanks to all who provided suggestions - they eventually helped me track this down.
73 de W0ZF