I didn't read most of this thread but I'll say only this:

MCH may not have Minne or Troubadour but do not underestimate the value of well-timed Dismantle. My group runs with a MCH and when we don't have him it's amazing to see the difference. On top of flat 6% DPS increase, the job is doing VERY well.

If anything, make it so that players in range of the turret get a passive +2% crit buff like the BRD songs (or make it dh, for variety).