I've only done this once but maybe I'll at least give you some things to think about...
1. Assuming you would sit in the middle of the space you will lose a lot of bass. I think it will be OK, and there are more important things to think about. You haven't really stated if it's more important the whole room is open (like for parties and casual watching) or you'd consider running a wall down to have an isolated home theater. For me it was important it was open and I'm happy to compromise to make this a rec room that doubles as a theater and not the other way around.
2. If you can see, is it drywall screwed right to the joists with no insulation? It's probably not worth pulling all the drywall just to insulate and put in channels. You might consider putting in a second layer of drywall over the first with green glue in between, probably the easiest way to go from where you are now. Nothing will do a great job with bass, unfortunately.
3. If you're particular about it and don't mind the research, and you have a contractor you like to work with I'd go that route. Unfortunately you might have to be pretty specific about what you want if he's not familiar with some of these things.
4. You could try a flat cable under the carpet, I wouldn't drill into concrete, or maybe you'll wall mount the speakers instead?
5. It's hard to put conduit into a wall under the drywall if the ceiling is also finished since the hole in the top plate is difficult to drill. It might be possible with bendable conduit. It would probably be easier to just fish cable where you want it to go. If you do pull the ceiling then yes run conduit while your'e there.
6. I'd start with what you already have. This is more a personal preference than anything.
7. No, 13 feet from 100" screen is pretty ideal. I'm 12 feet and planning on 100".
Really I think you should figure out exactly how you want the space and work from there. I think I'd start with the ceiling soundproofing. I have insulation and resilient channel and it's not amazing but much better than before. I think having someone put in green glue and a second layer would be the first thing I took a look at, if it's not going to work for you now.