I don’t post here as much as I used to... but thought I’d add my two cents.
You have a lot going on apparently, a very short time frame and a dog who would be very difficult to place even if you had all the time in the world.
All of those who are pleading with you not to euthanize this dog and giving you opinions about how lots of GSD people and police depts would love to take this dog are glossing over the very critical parts of your posts that indicate he has severe social and behavior issues. Police and military working dogs aren’t loose cannons with personality problems. A stable, balanced temperament is perhaps the most important criteria for these dogs.
Your dog needs to go to someone really skilled with dog behavior and dog training, who also have the time and interest in rehabbing a broken or defective personality. Those people already have dogs, usually. And those involved in rescue are full up, just with easy-going happy, balanced dogs who should be easy to place.
Unless you already knew of a good home in your personal contacts who know the dog, 4 days is just not long enough to search out, check out, and make arrangements that would not just further traumatize this dog. Taking an unstable dog and putting him on a plane to go meet some stranger at the other end sounds like a disaster to me, and an end much worse than your loving arms and a kind veterinarian... Perhaps one who would allow you to pre-sedate him before bringing him in, if he doesn’t cope with outside situations well... or better yet, would come to your place to do it.
I don’t think you would be wrong to euthanize this dog. It’s not the worst outcome. Ending up stressed out from a hastily-made decision to ship him somewhere far from the very sheltered environment he’s been in, sounds like the worse outcome, to me.
My two cents.
It would NOT be an option for me. This dog has lived in this world & is & has been able to cope. I don't believe that he is a hopeless case to train.
The owner did NOT say he is a viscious beast!!
Hair! “Welcome back”!
I appreciate your post.