Note in your first illustration, the upper and lower waves aren’t the same. They’re up and down mirror image. When you convert to mono, they cancel to a noise track.
Use the drop-down menu on the left > Split Stereo.
Select one of the tracks > Effect > Invert.
Drop down on the top left track > Make stereo track.
Now when you mix to mono, it should succeed. Another way to do this is Split Stereo to Mono and delete one track.
I have a cable that’s 3.5mm (1/8 Inch) Stereo Male on one end, and XLR Female on the other end.
Yes, but what you don’t have is a cable that converts an XLR microphone to soundcard. You have a Devil’s Adapter.
If you continue to want to record this way, this is the proper adapter.
It’s a mono 1/8" with the wire crosses listed in the pix.
The XLR microphone has two outputs. Pin 2 is the “main”, right-side up output and Pin 3 is the upside-down protection signal. If you have an interface or sound mixer that properly uses both, you can have 100 foot (30M) microphone cables with no sound damage.
Koz