I am no pro but here's some of my limited insight to your questions...
1) Could you expand a little on your future ceiling's construction? Are you planning on installing a completely separate set of ceiling joists, that will be fastened to your interior walls?
Assuming that is the case, may I suggest you fore go the use D/GG/D and instead use OSB/GG/D. With the added shear support the use of a OSB layer will give you, I don't see the need to connect your new internal walls to the exsiting floor joists. To be very clear... I am not a structural engineer, and/or building inspector, so my opinions are just that, and are not meant to imply that what I suggest is an acceptable way to do things according to building code. However, I am getting the impression that you are planning a room within room build, and I feel using anything to couple your efforts to the existing structure will hamper the end result.
With that said... What I am suggesting is exactly how I plan on to handle my room within room build.
2) If your build is a truly dedicated HT, I think you should concentrate on removing heat from your room. So that lends me to believe a high location for your returns would be best.
3) I think everyone here agrees that the more mass you have the better off your are. Where diminishing returns start to come into play...I don't know.