Indeed, but there's a chick and egg problem there. We put the plutonium process fully in gear before we even knew the average number of neutrons a Pu-239 fast

Indeed, but there's a chick and egg problem there. We put the plutonium process fully in gear before we even knew the average number of neutrons a Pu-239 fast fission would produce, when we had no more than micrograms or so of cyclotron produced stuff. Which was enough to work out the separation chemistry, ultra-microchemisty is amazing. I guess my point here is that the Germans realized uranium separation was a no-go for them, were trying reactor based approaches, and they might have rolled the dice that way and tried the plutonium path. I'm saying the pieces were in play to possible success, not that it was at all likely without 20/20 hindsight.