>>1466443You only leave webbing and a quick link when you use a knot block, and if that is too much money to leave outside, you probably shouldn't be doing technical rope work outdoors. A lot of rappels will already have both of those, although it is up to you to inspect and replace if they are damaged in any way. The locking carabiner is only there to back up the figure eight, as there is not way it can pass through the quick link, and the last one down watches the knot block and how it functions in order to determine if it is safe enough to rappel down without the biner block (if not, then they will rig it differently). The left side of the rope in the picture is known as the pull side, and you do not rappel down it, you pull it from the bottom of the rappel in order to retrieve your rope.
>>1466488The bowline knot is incredibly unsafe compared to a figure eight on a bight. It weakens the rope considerably more than a figure eight, it creeps and has a tendency to come apart when not being weighted. Double rope rappels are also not essential when using a knot block, as after the last person is down they can just pull the pull side, but a figure eight is a bulky knot and can definitely get stuck and make the pull hard, so doubling can be useful.
