Just have each recording on its own mono track, clap to sync them all up to the DSLR, keyframing volume shouldn't be too much of a hassle when you mix, then just finish in stereo.
Really the only thing that should be stereo in your mix is the music. Just barely fade other mono tracks if you want it for the stereo, but not too much or it's distracting.
a7s or GH4
I think that slot on the back is for the Battery Pack and other attachments, not a full sized SD card.