OK. I uncheked context menu integration in MPC-BE and pressed "Apply" button. And the context menu commands (both in root and in "Open with" submenu) have disappeared. Then I checked them back and pressed "Apply" button. All things returned to start state - context menu in explorer does not show root command and does show command in submenu "Open with". Dopus shows both commands but root command still shows error when executing.
You're still not doing what I actually said.
Turn off the two checkboxes.
Leave them them turned off.
Add the commands you want to the Movies File Type Group in the Opus file types editor.
Now I understand. It worked. Problem solved. Thanks.