I guess I still don't see how this is better than claiming an old, semi-retired plane on a routine exercise crashed due to mechanical failure. Could even have claimed it was a target drone, since such a thing had existed even before WW2.My guess is it's much more simple than that, Fe:
1) nuke loaded b-29 crashes (keep in mind the date, 2 years before soviets test first nuke there is likely strong desire to keep everything secret)
2) commander says oh crap, we'll need to clean this up and keep it secret
3) commander and his advisors find out that locals are claiming ufo stuff
4) commander says ok let's let that play out and this will blow over
5) later on they r getting too much pressure and attention and commander says we need to have a more believable cover story to make this attention go away, so they go with weather balloon