Well, cement panels seem great for soundproofing, but they're really not wise. Cement boards weigh about 100 lbs vs. 80-85 lbs for 5/8" drywall, so you get the beneficial additional mass. But cement board costs $20 per sheet vs. $10 for drywall and is a major bitch to cut and install. Also, cement board is porous like cinder block or concrete, so you'll have to really slop on multiple layers of paint to get good performance. Skip the headaches and keep it simple - stick with drywall.
The method you're thinking of using, sandwiching MLV with Green Glue, is probably not a good idea. Green Glue is designed to adhere 2 panels of equal density, so they will vibrate in the same manner and induce a shearing force in the adhesive and dissipate some of the noise into heat. Having MLV in there will just muck up the system.
As I'm sure you know by now, the best solution would have a decoupled wall, with insulation and lots of mass on each side. Having your wall panels rigidly connected to your neighbors (via the studs) is responsible for most of the noise you hear. Since it sounds like you can't achieve this, just install an additional layer of 5/8" drywall with Green Glue. I'm not sure what STC rating you will achieve - maybe it's shown on the Green Glue website. It will certainly be better than what you have now.