Am I able to decide where I want my products to be stored? Is it possible to have them stored exclusively in Germany?

Unfortunately, this is not possible. Amazon’s algorithm decides where your products are going to be stored. We do not have any influence on that matter.
Storing your products exclusively in Germany is not possible, either. With the participation in Amazon’s PAN-EU-program Amazon automatically decides in which countries your products are going to be stored. But the delivery of your products to Amazon has to be sent to a German warehouse only. In many cases we have to wait for a reply from Amazon. We can’t, however, influence how long this may take.