Wow Christine!!! For an 11 week old that you have only had for a couple of weeks, you guys are doing great!!! You should be giving advice instead of asking for it.
Very impressed with how well your little guy is doing, and I think your expectations at this time are a bit high - you couldn't really ask for a better pup for his age.
For accidents in the house - yes, pretty normal for his age - now each pup is different. People on here say that they have had completely house-trained pups at 8 weeks, but some pups may take a few months to be reliable. Think of an accident not as an accident from the pup's end, but from yours for not watching him closely to pick up on the signs that he needs to go potty. Keep doing what you are doing, it is exactly how potty training should go.
For fetch, to get him to bring the ball back, be ready with another toy, ball or treat in hand, and call him all exited and animated, and run the other way so he'll chase you. Once he catches up to you, you throw the other ball, or give him a treat if he is treat motivated. Training with treats for puppies is great way to lay down some foundational behaviours - in their brain, there is a strong association of doing an action and the action being paired by something very rewarding - down the road, you can phase the treats out, but the positive association in their neural circuits will still be there.
Three cups a day separated into three meals sounds fine. Watch him and how he is filling out and adjust amounts as needed. Pups sometimes go through growth spurts where they seem to sprout up overnight, and look thin and lanky - feed a bit more during that time - cut back to his regular amount when he fills out a bit and looks more visually balanced.
He isn't chewing on everything because he is teething, he is chewing on everything because he is a German Shepherd puppy! Teething starts at around 4 months, and ends at six months. For now, chewing on things is how intelligent, curious, pups who like to explore their surroundings interact. They use their mouth the way children would use their hands to examine something (and then pop in their mouth - so kids aren't that different from pups! - kidding!).
Another great way to deal with mouthing and land-sharking is to redirect the biting to a toy - when a pup bites a person, they are trying to interact and play the way that mother nature has wired their brains for interaction and play - I for one want to encourage a pup to interact with me, and develop and foster that desire, so instead of discouraging biting, one way to deal with it is to redirect to a toy. Always have toys at hand, or stuffed in you pockets, and when pup is trying to bite, whip out the toy, stuff in pup's mouth, and PLAY!!!! Play play play!!!! Playing with you is the BEST most FUNNEST thing in the world, but now pup learns that to have fun with a person, it is WITH a toy. With a bazillion repetitions, pups learn to bring you a toy to play with you. It does work, but be patient. This is a process or re-programming and shaping a pup's instinctual behaviours into specific actions. Takes repetition and time.
Leash walking is just fine for his age - they have such a short attention span! Be happy with the attention and control he does have at the beginning of the walk, then recognize that when he starts to wander all around, he has reached the limit of his ability to concentrate. Plus, leash walks are really boring for pups! Bring treats or a favorite toy with you, reward good walking with treats, and have little tuggy breaks during the walk to break up the boredom. If he cowers from cars, get his attention on you as a car approaches and before he reacts, and keep his attention engaged with tugging or some obedience drills rewarded with treats or more tugging, so that he can't even focus on the scary car, but rather on how much fun YOU are. Ignore his cowering or fearful reactions, and carry on in a matter of fact way - your energy and behaviour is the cue he is looking at for direction.
As for introduction with other dogs - be very careful to make sure that all his experiences with other dogs are positive. I would limit interaction to dogs that you know and know that they are safe and a positive influence on your pup, and avoid dog parks or random dogs that you know nothing about.
I'd say pat yourself on the back, you are doing a great job, and are well on the way to have a great pup!