the nav and the rad op would be in trouble since they have to climb over the spar. 
To do that they would be going backwards when their escape hatch is forward.
http://www.lancaster-archive.com/lanc_parachute.htmPosition    Location
Pilot    Seated on the left hand side of the cockpit. There was no Co-Pilot
Flight Engineer    Seated next to the pilot on a folding seat
Navigator    Seated at a table facing to the port (left) of the aircraft and directly behind the pilot and flight engineer
Bomb Aimer    Seated when operating the front gun turret, but positioned in a laying position when directing the pilot on to the aiming point prior to releasing the bomb load
Wireless Operator    Seated facing forward and directly beside the navigator[/b]
Mid-Upper Gunner    Seated in the mid upper turret, which was also in the unheated section of the fuselage
Rear Gunner    "Tail End Charlie" seated in the rear turret this to was in the unheated section of the fuselage and was also the most isolated position. Most rear gunner's once in their turret's did not see another member of the crew until the aircraft returned to base, sometimes 10 hours after departing
You forgot that the nav and rad op would also have to somehow get past the mid upper turret if they wanted to exit via the crew entry hatch.