I am sorry that you are feeling disappointed. Nonny sounds like a great puppy--I would try to love him for who he is. I also really doubt that the shelter intentionally lied. They are people, just like you and me, who do their best to try to identify some of the predominant breeds of the mixes that come in. They're not always right. It happens. Same with the age thing. They do their best to get a rough estimate, but it can be very hard to tell unless the puppies were born on site.
If you decide to get another dog in the future and are still set on a GSD, I would look for an adult dog at a GSD-focused rescue (that way, you definitely know what you are getting) or a reputable breeder.
Yep they are mostly volunteers and some could be truly clueless!