Your other questions - you're right, things are only simple AFTER you've made all the mistakes
This is why I rarely complain to people about redundant questions. They're not redundant if you're not sure what to do, and I doubt I'll ever have enough time to organize things here well enough so I don't need to repeat myself 
If you've left gaps between sheets the only deviation you'll need to do from standard mud/tape/sand operations is to use a bit more mud BEFORE the tape goes on - normal mud/tape operation is to put some mud on the joint, smooth it with the drywall knife, then dampen the tape and smooth it on OVER the mud, then a bit more mud and another smooth job. End result needs to be a level joint, or possibly a SLIGHT bulge so sanding afterward makes everything LEVEL and SMOOTH. By pressing mud INTO your gaps FIRST, you'll regain the mass in those gaps.
Offsetting joints and changing orientation of sheets from layer to layer ensures that there aren't two joints in the same area to weaken the isolation.
Taping and mudding when done - no problem. For inside corners, use some of the newer, thin plastic flex corner and do as light a mud in the actual corner as possible (press harder on the knife where it's smoothing the mud in the actual corner) Floor and ceiling moldings need to be floated off ONE of the adjoining surfaces - one way is to screw molding to the bottom RC on an RC type wall, keeping the molding about 3/16" away from the floor. For double-framed walls this isn't necessary.
Door - definitely trade the paneled door for a thicker, solid slab. Exterior doors generally have better seals, but depending on yours you may still need a separate seal kit intended for better sound isolation.
http://www.acousticalsolutions.com/prod ... l_kits.asp
Here's one plan for the ceiling -
). Butt-kickings are only fun when they work, and I'm glad it did in this case! 
