I've solved the issue and am posting to save others from my hours of pain.

Problem

On Windows 10,

Python is installed in C:\Python and added to Environment Variables, but will not properly run programs.

Typing python myscript.py in CMD does not run the program but does not produce any errors.

Typing C:\Python\python.exe myscript.py correctly runs the code.

Solution

Navigate to C:\Users\Username\AppData\Local\Microsoft\WindowsApps\python.exe in CMD and type del /f python.exe.

What was wrong

The file located at C:\Users\MyUsername\AppData\Local\Microsoft\WindowsApps\python.exe is viewed before the Environment Variables in PATH. This file just opens the download page for Python in Edge.

You need to delete this file or rename it, but the items in this folder are protected by Microsoft. You get Error 0x80070780 if you try to delete this file with file explorer. In order to delete it you need to navigate to the folder in CMD with administrator privileges and type the above to remove this file.

I hope this helps someone.

Putting this Q/A into the right format so the question does not look open/unanswered

Solution from original poster; tested and worked on my system

Solution

In C:\Users\*Username*\AppData\Local\Microsoft\WindowsApps delete python.exe. However, because these files are protected, chances are you will get an error message when trying to delete the file. In that case, you either have to use your administrator account to delete python.exe, or you call a cmd-window with administrator privileges and force delete ''''rm -f C:\Users*Username*\AppData\Local\Microsoft\WindowsApps\python.exe''''

About the solution

python.exe is not the python - executable one would believe it to be, but invokes the download page for python to be opened in the Edge browser or the Microsoft App Store. The question of "who would come up with such a sadistic thing" and "why?." shall remain unanswered at this point. Fact is that calling python executes this faux- python.exe before the PATH Environment Variables are invoked.