Cool project. BTW, check the build in my signature as it's almost the exact same size as your room; it might give you some ideas. I had more headroom (10' ceilings) and no "jog" in the wall, but not much different other than that.
Some initial thoughts. I would try to put the viewing position of the front row about 10 feet from the screen or more (for acoustics and viewing angles). Also, I would probably steer clear of a curved row of seats (they take up a bunch more space and your space is at a premium). If the second row will not be recliners, I think your 10" riser will be enough (because they can be closer to the front row). Non reclining front row will help with sound, viewing angles, etc. from the back row, too. Could be a great choice and keep that row off the rear wall.
You could make that riser go all the way across and get one more seat in that back row if you want to. My first theater had an entrance at the back, but I didn't have room outside the theater to build up stairs/risers to be able to come in at riser height, so I made the door to the theater open out, and made a cut-out in the riser inside the theater with a step up to it. So, you open the door, step in to a small landing area (same level as outside the door), then take one step up, then another on to the riser. Hope that makes sense. Then the rear riser could go all the way across and if you move the seating all the way over to the right (and maybe forward an inch or two), you could likely get 3 seats across there.
Back to the riser, 10" is too high for a single step up (code requires about 8" or less, usually), so I would either lower the riser (if you can still get the angles you need...draw it out) or add a step up onto it. BTW, the step doesn't have to stick out from the riser, it can be embedded in the riser (again, see how I did my last theater steps as an example).
Use backer-boxes if you cut holes for speakers. Use backer boxes if you cut holes for lights.
On your question about insulation in your new ceiling...it's important to understand that the insulation is doing very little (next to nothing) keeping sounds in or out. What makes it important is damping sound that is inside the new boxes you're building (the space between your new drywall, and the floor/drywall on the other side of the ceiling/wall). You don't want big, empty spaces for sound to resonate, so fill empty spaces with any type of insulation; the fiberglass stuff just happens to be the cheapest. And it is very cheap...so I would just loosely fill any spaces you create building the ceiling. Also, the technique that DaveClement outlined above of using drywall with green glue in between existing joists is also something I did in my latest build (and it inspired my avatar) if you want to take a look at those posts in my thread
, you'll get an idea of what that was like to do. I can't comment on the effectiveness since I did not do a comparison study (obviously since I didn't want to completely build my theater twice), but I was happy enough with the results.
In fact, I agree with basically everything DaveClement said in that post above...so do that stuff.
I also used a power bridge in my last theater to save the bulb...worked like a champ.
+1 on your planned screen size (I used that size, also). I would not recommend DIY unless you just love doing it; I used an Elite Screens fixed frame screen, 120" 16x9, and it was fantastic (and very cheap at around $265 now). It was well built, the material was great, it went together nicely, looked fantastic, etc. I just don't think it's worth DIY anymore these days with companies like Elite Screens and Silver Ticket around.
Wiring is relatively cheap, so I would run at least coax and speaker cable to all possible future sub locations. The obvious spots to try first, like you said, for dual subs will be either front middle and rear middle, or front left and rear right.
I like the rack placement that you're thinking of. Keep it out of the room if you can (again, I've had a theater with the equipment closet in the room, and I've had it with the rack out of the room...the latter is preferable in my opinion). You don't need blinky lights and extra heat load in your theater. BTW, you could consider putting some shelves in the closet area to hold a game console and Disc player that exposes the front of those devices to the hallway...that way you don't even have to go into the equipment room to put a disc in. Could look pretty cool and be functional, too.
That's all I can think of right now...but like I said, looks like a great project. I'll be following the progress. Good luck!