the FN64 was a pretty good turret, probably the third best belly turret after the Sperry and B29 GE remote controlled units, however all these proved to be pretty useless at night, it wasnt the guns it was the positioning, being under the bomber meant that was pretty impossible to see an attacking fighter, those bombers fitted with the belly gun show no better survival rates than those without.
I think the decision was made to delete them from production was based on feedback from crews as to thier usability.
As to mainforce using upper turrets, I dont believe that mainforce had any such restrictions, it was purely how they were delivered.
A point about fared over turrets, I did forget one role in which they were used without turrets, all the PR1 photo recon birds were fited without nose and upper (not sure about rear) maybe he means these birds (would explain why they were carrying bombloads similar to b17s flares were quite light compaired to HE