I always like my main cook surface to be about counter height so aim for about 36 inches from the ground. This makes my upper rack at about 41" and this seems to work well. It's fine to adjust as for your preferences but don't get too high that you you can't open your cooker door.
Sorry, not sure what you are asking in your second question.
For the third, I would think 12 inches is a pretty good place to start for your shadow plate.