Having given this some more thought this morning, there’s an “ah, but…!”, isn’t there? The Manual Duck is “destructive” editing whereas the Envelope Tool is “non-destructive” editing. It is always possible to make a change to the Envelope Tool settings at any time provided the project has been saved. Once the Manual Duck has been applied and the project saved, there’s no easy going back: the change has been committed. Unless, of course, a second copy of the music track exists either within the project or outside of the project.
I still see this effect having value for doing a quick, rough mix-down of voice-over and music. But, for the moment at least, I think I need to ensure that I always “Don’t Save” the project when it’s in a “Manual Duck Quick Mix” state.