hopeposting

This is the same kind of logic that would say 9/11 is fake because one of the terrorists' ID cards was found unscathed in a New York street.

I'm not saying it happened, but your skepticism is way out of proportion.

• The filling issue is an obvious problem, which is why any Czech trying to sabotage it would have spent the first couple years of their enslavement trying to to come up with a method that works. Maybe the factory made its explosives in-house and the explosives guy was in on it too, instead making inert chemicals with the same raw materials. Maybe the explosives where rendered inert through a chemical process. Maybe the explosives were smuggled out of the factory to empower resistance movements and exchanged with dirt.
• When locked in a factory and ordered to make a certain amount of explosives, not making those explosives but making duds instead leaves you with a lot of spare time. If you're already supposed to be a line worker making thousands of the same thing, why not make thousands of similar letters expressing your hope of rescue?
• The shells would have been more solid than explosive shells, being filled with inert shock-absorbing dirt. Many of them would have exited the craft, but some would naturally have gotten stuck because the point of cluster munitions is to be birdshot.
• It's good military-scientific practice to study the weapons of the enemy, especially if they don't do what you expect. British military intelligence was very thorough, down to using novel statistical techniques such as the German Tank Problem. Explosive munitions not exploding is a definite curiosity worth investigating.
• No, the Nazis did not have good quality control. A lot of the popular conception of Nazis as technological geniuses comes directly from Nazi propaganda, repeated by American propaganda for the sake of demonizing the USSR and trying to justify Operation Paperclip scientists like Von Braun being given a warm welcome and getting rebranded as a hero.