So, you visited a real B-17 and found the mysterious bulkhead and the small door? I was wondering if the one on the Boise Bee was an aberration of some kind. I’ve not found it anywhere else. My uncle served as a tail gunner in a B-24 which has much more room in the tail tunnel for the gunner to escape and still my uncle had to leave his chute forward as he could not take his tail gun position with his parachute on. He left it forward by the exit door. So I doubt if parachutes were generally worn by tail gunners in a B-17.
Actually, I do not remember if I saw the bulkhead and door, but it does make sense to keep the tail gunner cozy while at his station. What I do vividly recall was marveling at how little space there was to get around the tail wheel structure, as it looked impossible. I was 50 lb lighter back then too. 
Look at the tail gun position from the outside. It is literally 18 inches. That and the ball turrets would have been nightmares.