Hi,
If I understand you correctly, you want to build a mix from FOH surface, but control that mix from Broadcast surface. Is that correct? In that case you are correct; You can just press MIX on that mix.
The only way (that I know of) to have the Broadcast surface to be ‘independent’, and have it “locked” on its own Main LR mix, is to have a second mixrack (DM0 eg.), with a GigaACE card.
I haven’t seen or heard of a way, to ‘Lock’ a surface to a specific mix unfortunately.
To answer the PAFL question:
” due to the fact that FOH is not using those inputs/has them turned all the way down ”
Select the channel, press Routing, set them to OFF on Main LR, and set the faders to 0 on the Main LR layer. That way they can be ON, turned up, and not be active in the Main LR-mix on the FOH console – and you can PAFL them on the Broadcast surface.
Personally tho, I would always prefer to have PAFL prefader, but that is a different conversation..