I bow to your superior knowledge. Though if the Germans have a gun-type Bomb, could it work? Your explanation is slightly ambiguous, i.e. did it take years to make even a gun-type "laydown" Bomb? AFAIK, implosion bombs completely replaced gun-type almost immediately. Also, live testing could be performed with dummy bombs.
Implosion bombs were set for laydown OTL, but the parachute problem for them did take far longer than expected, with drogues to slow till the main chute would release.
All while retaining accuracy, even given the area of effect the bombs had with the expected increasing yields of the early/mid '50s. The early bombs had drag plates that failed during tests, causing the bomb to go off course.
Yes, you can test with dummy casings, as the US had done since early '45, when the casings were done months ahead of the real thing.
The 509th did a lot of drops with 'pumpkins' filled with HE only