Just so I can understand that all correctly because I'm interested in the option:
As soon as the pre-order goes live, I should be able to make a try-in reservation for the Apple Store in Berlin. There, if I like a model, I can say 'OK, I'll take that one.' and then pick it up on the 24th of April?
Nope. They will tell you to go to apple.com/store and order your Watch there. There won't be any pickup in the stores.