Hmmm. Sounds like you're being "Penilely Encompassed", or simply dicked around with, as we say hereabouts...|
I'd ask to see someone higher in the chain of command to ensure you are properly prepared to get exactly what they require, and ask on just what grounds they detained your package and forced you to open it, how many other Chinese imports are they holding?, and then inquire regarding how many more they had intercepted coming into Germany and why the declaration from the others that someone had no doubt produced by now didn't apply to your model? I know there are several others posting in this forum that have indicated German residence.
This may just be a local official making more of something than need be made, perhaps escalating the issue above their jurisdiction or mentioning that you feel that it may be necessary to escalate this further in order to get the precise information you need to clear your package might get them to break loose from their stance rather than requiring you to jump through hoops if it means bringing their management or superiors into the matter... You never know, if you're polite and earnest in that you just want to make sure this gets resolved in EXACTLY the manner required. If they attempt to dissuade you from escalation, you've got them, just lay it on a little bit at a time. It could just be some clown who likes the idea of playing with your toy while you wait.
US customs has never even opened the box on anything I've ever ordered from China/Pacific Rim... Tablets, microcontrollers, electronic parts and modules, etc.