I’m obsessed with trying to figure this out, so I want to put this obsession to an end. I’m not quite sure why, but I feel compelled to do this.
For those who are unaware, to make an audio silent in mono but play in stereo, you do this:
Simple enough? Now, I want to do the opposite where the audio is silent in stereo and it plays in mono.
I’m optimistic for your reply. 
The original process isn’t quite right. It’s not stereo any more, it’s two track mono. It’s also missing the right channel and if you listen in headphones it may make you seasick because the dimensional clues will be off. Like listening with the performers behind you. So it only plays in stereo if you don’t mind all the damage.
I don’t know any way to make it work the other way around.
Koz
By default, if you do not have the capabilities to play stereo (both L and R channels) you would revert to the Left channel which is your mono. I’m not exactly sure why you want to do what you want to do but here’s the thing. Stereo is two channels. By making audio silent in mono but play in stereo, you were playing the empty channel in mono which was silent but you could still here it in stereo because you had the other channel with audio in it. Now, you are trying to make audio silent in stereo, but play in mono. You can’t because you will be deleting both channels of audio and not have anything to work with. Give it a shot though.
Simple enough? Now, I want to do the opposite where the audio is silent in stereo and it plays in mono.
I’m optimistic for your reply.
There’s no way to do that. Stereo-to-mono can create silence (if you don’t have true stereo) because mixing is done by addition and when you invert you are “adding a negative” which is the same as subtraction.
“Mono-to-stereo” is done by duplicating (multiplying) the mono track so nothing can be subtracted (and again you’re not getting true stereo).