Rebuilding 5.1 Audio Post room after flood....

Hello all Long time reader, 1st time poster. So here is the short version of my story. I have two 5.1 mix rooms for television promos on the top floor of a 17 floor building in Manhattan. They are in the middle of a corporate office surrounded by normal offices and cubicles. Back in the beginning of January a water pipe blew above one of my rooms and flooded it. It was literally raining inside the room. Anyway, it ruing the room, the icon, and a bunch of other stuff. In the end the insurance company has decided that they would like to rebuild the room across the hall since the water treatment plant for the building happens to be right above the current room (would have been nice to know the first time we built). So what I need to do is build a room that will provide adequate sound proofing for an 85db mixing level so that the rooms that are literally on the other side of the walls are not bothered. All the while trying to maintain descent room ratios and such. We will be taking 2 rooms, blowing the wall out from between them, building new walls, and replacing the hanging ceiling with a solid ceiling. Some of my biggest questions are: 1 - Do I leave the existing interior sheetrock up or do I tear it down? If I leave it up, I create a 3 leaf wall, the interior rock for the new walls, the existing interior rock, and the existing exterior rock. So is a 2 leaf system (2 or 3 layers rock w/GG, steel stud, air, steel stud (existing wall) single layer of rock) or a 3 leaf system (2 or 3 layers rock w/GG, steel stud, air,1 layer rock, steel stud (existing wall) single layer of rock) going to be better in this situation. 2 - How much would a 3rd layer of rock get me on the new interior wall? Since I cannot add more sheetrock onto the existing exterior wall because I would end up in someone's office, would I benefit from adding a 3rd layer of rock on the interior new wall? The new wall is a separate piece of steel track an inch or so apart from the existing wall, with the studs staggered from the existing wall, with Green Glue between the layers of rock. The new steel track will also be mounted on Kinetics Wallmatt to help separate the walls from the foundation. R13 between walls. The walls are built all the way up to the slab. 3 - There is a support column right rear of the room. Do I build the wall to follow the existing wall, creating some funny angles, or do I build the wall straight across the pillar and leave a 2ft air gap in between walls? Other info on the project.... The floor is carpeted with acoustic rubber matting underneath. It sits on the concrete slab which is 20" thick to the next floor. I'm not too worried about bothering them. The ceiling will be hung from the existing slab. I was not planning on using any sort of isolation hangers unfortunately because I think the insurance company would balk at the extra cost. My supervisor also wants us to use a type of high density soundmatt in the ceiling as well. Not a big fan of it, but he wants it. Wall height is 8' 10" to the current hanging ceiling & probably about 11' - 12' to the slab. Due to all the stuff up in the plenum currently I am guessing the new ceiling height will be about the same as the current. I don't have a lot of control over the HVAC. We have separate water cooled systems in the room that are mostly quiet as well as HVAC from the building. For the existing duct work I am hoping to be able to put some dampers in to slow the air, put a few 90 degree bends to eat any sound coming in or going out, and maybe put a larger opening into the room. Same fore the return into the plenum, lots of bends. That's what I have so far. I am sure questions about acoustic treatment will come up later, but right now I need to get design down for the contractors to placate the insurance company. I know this will not be a perfect mixing environment, but I would like to make it as good as I can with what I have. Any and all help will be really appreciated. Thanks in advance... mark