Pre war Wellingtons/ Whitleys had 2 pilots as did the later Stirling/ Halifaxes and Manchesters. The Hampden and Blenheim had one,as per design.
For Bomber Command to expand, it was decided in the spring of 1942 to economize on aircrew and drop the 2nd pilot.
A number of Bomb Aimers were failed Pilots and these could be of use in an emergency.
The discussion re .50 calibre has been had on here before a number of years ago. The use of them had been put forward pre war,the reasons for non- implementation were supply issues until 1944. The Rose .5 turret was a private venture fitted to 1 Group and some 5 Group aircraft from late 44.
We could then go into Freeman Dysons research into doing away with turrets and armament altogether, which he computed would add 50mph to a Lancasters top speed. This was however not acceptable to 'bomber doctrine'.