Hi
Ok I don’t have one if your mixers but it seems from your post that you are making a big presumption about this cable being an output at all.
Try plugging the cable you are hoping is an output onto say your Mac headphone output and malting some audio from the Mac at Mac volume
If you see this audio on any mixer input / meter then that cable is indeed going to a desk input, so no good.
Personally I’d think it unlikely that a mini jack cable would be an output?!?
You really need to get access to the rear!
Peter