Welcome to the world of a German Shepherd puppy.....positive reinforcement of good behavior and at times a stern hand is required. GSDs certainly have a high energy drive and at times I found with our pups their prey drive exceeds their food drive, which makes it even more "interesting".
The story you paint of the walk somewhat makes me chuckle as our current pup played the same games, especially with my wife. I found a pocketful of treats would help keep Kaia on the straight and narrow, when she was walking with me on the leash, I would treat her quite often. I might suggest a prong collar as I have found it to be more effective than a choke chain collar and supposedly more "humane". The forging problem is probably something best cured early rather than later. I was taught a method of " back, pop and pivot" by a skilled dog trainer which worked amazing well. If Kaia would get too far out I would say "Back" then pop the leash to give meaning to the word "back" and then pivot in front of her so we would be facing each other....and then start again. Believe me, we didn't get very far on those first few walks but in short order we were a team walking together rather than struggling against each other. Soon enough, all which was required was the verbal command "back" and she would fall right into place with her head near my hip...with a loose leash. We walk together wonderfully now....as one.
Bait and switch worked well for me with Kaia as far as getting her to drop something she was not supposed to have.....but, yes the "hey look at me I have something I'm not supposed to and you can't have" games she played for a spell did test my patience. I started using the words "leave it" and then follow through with a super premium treat or toy to have her come to me..with or without what she wasn't supposed to have. I wonder if perhaps the "right of possession" is somewhat the mentality going on in a dog's head when they exhibit this behavior of running away with something they shouldn't have or maybe they are just having fun, hoping you will chase them.
The jumping took care of itself as a low key approach to our greetings after being apart became the norm. If our pups over the years wanted to greet us with jumps and joy, we learned to more or less ignore the dog's exuberance ( which was hard to do). I would simply ask if they wanted a treat and then get them to sit or lay down upon our arrival. Our Kaia is a world class leaper when we play and that is okay by me, it's her time to get her jumping desires out and I always am careful knowing that she can easily leap high enough to get near my face and head.....but we turned the leaping into a game of sorts ....only when it was permissible.
One last thought, and I know this will be unpopular with most in here but when our puppies were old enough to know not to bite with those needle-like puppy teeth but persisted....I'd bite them back on the muzzle across the top. Now, whether or not this is frowned upon, all I know is it worked. Personally, I do believe in order for this approach to work, the human doing the biting back should most likely be in the driver's seat as a dominant in the pecking order otherwise one might find the situation escalating. I am certain there are better ways for the majority of folks to cure a dog's nipping problems, so I'd look for a better solution rather than my approach but it has worked numerous times.
The best part you can look forward to is what you honestly stated at the beginning our your post..." I am starting to think he is smarter than I am." Shepherds are extremely intelligent and this will come to be one of the best parts of Samson's attributes. My experiences seem to indicate as time passes they are quick to learn what is acceptable behavior and have a strong desire to please their human leaders and perhaps that is the key....as the old adage somewhat goes....a dog either leads or is led....and this has nothing to do with repression just a simple understanding between dog and human which manifests itself with respect being a two way street. You are the steward in this relationship and with this stewardship come responsibilities, you must lead the dog with your intelligence, patience and kindness...I know I Know....I bit my pup....LOL.
Most everyone who has every owned a GSD from a puppy has certainly experienced some of what you are dealing with now but at the end of the day, they mature into one of the most loving, loyal intelligent furry friends a person could ever wish for......
Oh, one other thought. Seeking out a skilled dog trainer is well worth the monies spent. I believe what you will find is the dog trainer teaches you how to interact with your dog and once you see how easy it is, you will almost feel embarrassed that you couldn't figure it out on your own.