Yep it was a guess. I hate it when they don't include schematics but only nerds like me read them so I guess it's a waste of effort to publish them.
Do you have an eternal effects unit? You could use that for headphone reverb. Just split the FX Send and route it to both the reverb and the computer.
If it was my mixer I'd be tempted to open it and hard wire the socket so it still fed the internal effects. Strange design choice of theirs because you can just turn down the FX To Main if you don't want the internal effects so sending to both wouldn't be a problem.
Honestly, opening it up and "making" a fix was my first thought, but I can't find anyone who has done it. While I do repair computers for a living, audio is mostly overlooked. I could actually do all of this easily with software, vr cables, etc, but the latency is terrible. For now, I found that SOME 1/4" adapters will split the mono male signal to stereo female 3.5mm, but not sure exactly which ones yet. The one that came with my AT headphones definitely won't do it. Still looking around through reviews.
I don't have an external FX unit unfortunately.