Good questions, and I am sure that you will get some great replies. You can look through the AR discussion forums here and see setups in the picture threads.
Remember--how one person likes to set their AR up can vary differently from another's. That is the beauty of the AR.
Sling--I recommend an adjustable two point with QD buttons. You can mount the sling anywhere from the brace to the front part of the rail and anywhere in between. Find the location and combination you like best.
Red dot--whichever one you go with; near the center of the upper receiver. Pretty simple.
Backups--at the end of the upper receiver above the charging handle. Front as far forward on the rail as possible, and depending on if it is a flip up, enough room to lower and protect it.
Light--at the 3 or 9 position on the rail, depending on if you are left or right handed.
I hope that helps!
BP