I think the allies would have used the bomb in Europe if they had enough of them to use. After the war in the 50's there were thousands of warheads all over Europe ready to use in Europe on the Warsaw pact forces.
The B-29 use would be a problem like it was in the Pacific. The 21st Air Force was created because so many Generals and Admirals wanted to have control of the B-29 for their own pet bombing projects and alot of "interservice politics" were fought for control of the B-29.
I don't know much about the engineering of the shape for A bombs but Little Boy and Fat Man were very different in shape so perhaps they could have shaped one to fit in a Lancaster for use. (or maybe even a Halifax
)