Thanks for your time. Indeed, I am able to use "Flexible Track Routing" as Avid calls it here:
https://youtu.be/AdAay-UVP4c
You can see from my screenshot, I'm able to print a 5.1 mix and downmix it to a stereo mix at the same time.
screenshot1 (on Flickr)
Also the Sends off my 5.1 buses get converted to stereo automatically, so that all works great.
It's the preference between Classic and Modern that is acting up here it seems. When I switch to Modern, Pro Tools ignores my Master Fader. The Pro Tools Reference Guide explains the differences between Classic and Modern, page 123, but it merely changes the math involved, I don't think there is anything functionally different that would cause this.
For what it's worth, you can see I actually have two Master Faders, one for each set of speakers (5.1 Genelec, 2.0 NS-10's). I've now tried deleting the stereo Master Fader and switching to Modern, but same deal, it ignores my Genelec 5.1 Master Fader.
The other weird thing is when I switch back from Modern to Classic in preferences, the Master Fader is still ignored until I quit and relaunch Pro Tools. That's why I think it's a bug.