3) None of the film aircraft had machine guns actually fitted in the nose perspex. But photos of the real 1943 ‘Memphis Belle’ shows two 0.50s mounted at different positions in the nose perspex. I have also seen photos of some B-17Fs with twin machine guns projecting from the exact centre of the nose perspex. Were either of these factory modifications or were they done in the field? Also did the twin mount use 50s or 30s?

I think the mounting for extra nose guns was included on certain productions of the B17 at a factory level.
However the twin gun modification were done in the field usually utilising 50 calibres.