are a very good place to get all this information.
I would contact the ADAC or similar, they should be able to answer all those questions. http://www.adac.de/
Maybe it makes sense to become a member, then usually there would be free legal advice included in the membership, and other benefits (at least I have this with ARBÖ in Austria).-is it necessary to maintain insurance during the storage time?NO
- assuming the car is in private storage, not sitting on the road somewhere.-is it necessary to maintain registration during this time? NO
- again assuming the car is in private storage, not sitting on the road somewhere.-is there a fee for storing a vehicle for a non-citizen/non-resident?
None that I knew of.-is there a maximum length of stay for the vehicle of a non-citizen/non-resident?
You may have to get German registration after a certain time, that means you have to have a German address. Automobilclubs should know. The question is an interstig one, if the car is only used for a short period every year.