>>64089471
It's not logistically simple to relocate an entire bombing group, and the sudden presence of B-29s in Europe would have attracted a lot of attention to what was a critically secret operation. Bombing Japan instead of Germany would have still been preferable to the US because the pieces were in place to support such an operation.

In all likelihood, if the US wanted to drop a nuke on Germany at the time, they would have opted to use Avro Lancasters for simplicity and bomb development would probably have continued along the lines of the original "Thin Man" gun payload rather than the "Fat Man" payload that was optimised for B-29's shorter bomb bay.
However if bomb development still went as it did, the Lancaster would have still been able to carry Fat Man externally in the same B.Mk.I Special configuration used to carry Grand Slam bombs.

Because of B-29s development problems, Lancaster was a serious consideration for the USAAF at the time but wouldn't have the range to carry out Op Centerboard I as-flown by B-29 from Tinian to Hiroshima (it would have had to fly from Okinawa).
But Berlin was certainly within range from England for Lancaster.