Wow, this doesn't sound like hip dysplasia at all. Wobblers? Exercise induced paralysis? Slipped disk -- really young for that though?
The thing is, with hip dysplasia, the long bones do not fit in the joint correctly. So, there is pressure building up between the bone and the joint. Arthritis develops, calcium deposits can build up in there, and bony matter rubs against bones instead of cartilage, and gets progressively more painful. I just don't think the puppy would not be able to get up off the floor one day. That is more like a slipped disk -- injury, where the dog, through inflammation and pressing on the nerve, the back legs are unable to move. And it can happen higher up too.
Ununited Aconeal Process -- elbow dysplasia, is where the bones do not fuse together by 5 months of age. This will cause limping/pain. Hits dogs young. Often requires surgery. Can be genetic, can be caused by an injury. I have some questions about that. GSDs are a large breed and they develop slower than small breed dogs, but it seems across the board that the bones should fuse by 5 months. I have a bitch who will be 7 in March. She was given back to me just under 6 months, because the owner's kids caused an injury to the dog. They had her x-rayed, and they said UAP. Fine, I took her back with her x-rays, and her leg all swelled up. I took her and the x-rays to my vet, and she said, let's wait and see.
I let her heal. She healed. She has not limped since. At all. Will she get arthritis there as she ages? Probably. But she would have if I operated too. So, I am in the camp that does not want to rush into operations.
I am really skeptical about Hip Dysplasia causing what you have going on. Elbow dysplasia -- maybe. Obviously not in the back. The two are not related. A dog can have both, but there is no reason to think that if the dog does have one, he will have the other.
I think the dog has enough symptoms at this point, to take this dog to a specialist, have x-rays done and find out exactly what is going on with the dog. There are so many things: spondylosis, spondylitis, DM, Lymes Disease? Wobblers. And probably others. There is a possibility, because the breed is so often associated with HD, to jump to that conclusion.