1. IF it jams, you are fucked. The most simple and most sophisticated construction can jam under the influence of sand, salt, mud, dangling ends of rope, a crash impact with a rock, you name it. The only thing that can't jam is an open system like solid open hook.
2. true one hand use.
I'm not sure what you mean by 'true one hand use', but as for
jamming, you would have to be real foolish to have only the shackle
release to disconnect you from the kite. If you use a shackle, you
definitely need a quick release (at at the end of the CL (chicken loop))
As for safety, when you're being dragged at full speed while on your
stomach or on your back, the difference between the shackle and
the 'solid open hook' diminishes, and you have to rely on your
quick release anyway, because of the high tension in your front lines,
you may not be able to pull down and unhook from the CL anyway.
Keep a knife handy, and practice often grabbing for the quick release
to reduce your reaction time.