I'm pretty sure Hitler risks nothing unless he happened to take a stroll in the gardens at the moment of the strike. Radiations wouldn't be an issue either, this is an A-Bomb, the amount of fissile matter was very small.
I am aware of that yes.The nuke was developed to defeat Germany, not to defeat Japan. They just managed to defeat Germany before they could use nukes. If the Manhattan project had finished in 43 you can bet that they'd used it.
Also, the US had a concept to defeat Germany and it semed to work. The concept to defeat Japan was much more in doubt. Even if downfall would have suceeded the loss of life would have been enormous and the chance of it to fail wasnt that small. Certainly more risky then Overlord.
Would the Fuhrerbunker have been enough to protect H though (from the immediate blast I mean, not the radiations)? If not it's a good chance to finish him and possibly have someone more interested in surrendering take his place.