I hate to state the obvious, but your position is weak due to the time that has passed since you first received your case.
I wonder if a German customer bought, damaged and returned the computer case before it was shipped to you.
Did the original Silverstone retail box look like it had been previously opened before you opened it?
Was the computer case in a protective plastic bag?
Did any protective foam look damaged in any way, possibly by someone being careless when they might have removed the case?
Search the web for "un-boxing" videos for your case model.
I've seen many un-boxing videos for all types of computer hardware, sometimes from well known websites.
Some review sites have detailed pictures that show how good or bad the packaging is for the product that they are reviewing.
One thing that might significantly help you is to have someone local inspect the case to corroborate your findings. This inspector would need to be qualified to determine how the damage probably occurred and if any of the damage was due to manufacturing defects.
You sound reasonable and it's important that use a firm but polite approach at all times.
Losing your temper won't help you in the long run.
You need to contact any consumer protection agencies where you live, in the city where the seller is located and/or the location where Silverstone is located to see if there is any way that they can assist you. There may be public or private agencies in some of these jurisdictions that might be able to mediate an agreeable solution.
Don't be too quick to negotiate, you want an undamaged computer case.
If needed, you can compromise later.
I don't envy you, you'll be dealing with people from dissimilar cultures.