I'm not a historian, but I am a paratrooper, and I'd bet that collisions in the air arent as likely as you'd think. At least not collisions that lead to catastrophic failures. As for the second part of your question, American paratroopers carried a reserve chute, while Brits didnt. Feasibility of reserve deployment varies greatly with the jump altitude, so having a reserve doesnt always mean that you have enough time for it to matter.