I think you're missing the point that, as designed the B-29 couldn't drop the bombs....and at the time, it wasn't known *if* Boeing could re-design the aircraft to do so, due to the spar mods, hence the contingency plan for using Lancs.
I'm not sure though that the contingency plan to use Lancs ever included using RAF crews though......?
The B-29 Silverplate could and did carry both of the atomic devices successfully. It is not generally appreciated the great difference in sizes and shapes of the two bombs one uranium based and the other plutonium. The Trinity test and the two bombs were all detonated in 3-4 weeks. The Group on Tinian had been training for a long period and delivered the bombs within days of receipt.
There is a wealth of information on the admirable Atomic Heritage Foundation website with many unique photographs.
I am an admirer of the program maker and enjoy his programs but this particular one seems to be fostering yet another urban legend.
Apologies if I have expressed this clumsily