Re: Try this hypothesis on for size.
<divide [leaving one part in Lunar
orbit with a transmitter, and the other
part (the stealthy-nuke and it's propulsion system) to head back to Earth]
A few days later, that stealthy 400KT device
Stealth is much harder to achieve in vacuum than in the atmosphere. Your scenario basically falls apart right there. Which is also the reason why putting nuclear weapons on the Moon is pointless. Any launch would be detected, and the target country would have three days to retaliate.
Also no orbit around the Moon is stable for more than a few month, but that's a minor point.