Mixing in mono is only done as a reference. You use it to check EQ separation without the extra "artificial" separation caused by panning. You don't leave it that way.

1. Collapsing the master to mono is sufficient
2. The assumption is you'd put the master back into stereo, where you can pan things wherever you want.
3. You should never have to do that
4. Reverb is tough to deal with in mono. Mixing in mono is really just to check levels and EQ. I wouldn't make reverb decisions in mono.
5&6. If the final song is in stereo (as 99% of music made after 1965 is), then you'd obviously master in stereo. Mastering is where you are making the final decisions. Whatever comes out is the song.


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