First of all, you must be doing something wrong with your kit if it is riding up too high or low. To adjust the trim, you position the tank higher or lower, but always placed so that you can turn on and off the valve by reaching back.
There are several guidelines on the web for how to properly adjust the backplate height on your body, most of them are on DIR (i.e. Halcyon) sites. Do a google search.
If the rig isn't comfortable in the water...well, I dunno about DR, but all of the plate combos I have used (generic and Halcyon) have been rock solid and comfortable.
Also, the wing shouldn't trap air, if it does, then forget DR altogether, start over and buy a Halcyon Pioneer or Oxycheq wing. There are enough threads on the board to probably to have figured that out before. I would also recommend ditching the DR harness and get a proper Hog harness. Doesn't even have to have the fancy "H" logos on it.
Getting the harness adjusted properly should not be a lot of work. The trick is to snug it up tighter than you think is necessary so you get only a couple fingers under it on land. In water, things loosen up a bit. The crotch strap does wonders for comfort as well.
You might ping
Rich Lockyer on the board, he's very knowledgeable.
Best of luck,