I’m not familiar with the audio editing in FCP as I learned surround mixing in Pro Tools. But to answer your other questions:
The standard configuration for surround sound setup is LRC, Lfe, Ls, Rs setup. To do this you would go to your sequence settings and go to the audio outputs tab. You will want to designate your 6 outputs and set them to dual mono. Then you will go back to your timeline, and you bring in your 5.1 mix as follows:
Left channel on 1
Right channel on 2
Center on 3
Lfe on 4
Left surround on 5
R surround on 6
There is an icon on each track that is to the right of the lock icon. You want to click on that and designate each channel to go to its corresponding output.
As far as 5.1 headphones, the best suggestion is to mix on an appropriate monitor configuration.
-henry
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