PN,
You're quite correct about the escape hatch located under the Bomb Aimer's position. However, I understand it was relatively small, especially for a crew member togged up with flying gear, mae west and parachute. It was also still a struggle for the Nav and Wireless Operator to get right the nose of the aircraft as I understand their crew positions were located in a relatively central position of the fuselage, compared to the Halifax.
To put things into context, the stats for shot down bombers record that the number of men surviving a Lancaster crew averaged 1.3. The number from a Halifax, again assuming a 7 man crew, was 2.45. One reason for the higher number of Halifax crew survival rates may have been the Nav and Wireless Operator were located in the forward fuselage, under and ahead of the cockpit, and much closer to the forward escape hatch.
But whichever way you look at it, I can not imagine the horror of being in that situation, and I think night must have only have made the chances of survival even more slim.