I've finally got this working with anaconda, and figured I'd share in case anyone else wants to try this and searches through these forums for help.
I uninstalled Anaconda, downloaded Python 3.8 directly from python.org and tried running the plugin (using the exact same code as above, with the DLL fix) via the command line as you suggested, but was getting the same error message (this time it just said 'the specified module could not be found'). I tested some earlier versions of Python as well (3.6 and 3.7), and got ' "errorMessage": "module 'os' has no attribute 'add_dll_directory'"' - I guess this was only added in Python 3.8, and without the add_dll_directory line I still got the "module not found" message.
The fix is to edit the system environment variables on Windows (Control Panel -> System-> Advanced System Settings-> Environment Variables). Under user variables, change the "PATH" variable to C:\Program Files\Gwb (not "PYTHONPATH", which should still be C:\Program Files\Gwb\src"). This did the trick, and now the plugin successfully runs both directly in a python terminal, and in my Jupyter notebook. The "add_dll_directory'" step is no longer necessary, either.
I am guessing since this was not the fix in your testing, it has something to do with how Python interacts with my emulator version of Windows 7. But in any case, I wouldn't have figured this out without going through these others steps you suggested first, so thank you very much for the help!