Why did you get a active breed like a GSD if you work these long hours and days? That dog goes crazy from boredom and acts out when you are home. Can't you arrange dog sitting if only for half days?
Really? 8 hours a day at work, with a break at lunch at times is not too long. Most people are away even longer hours than that, and the dogs are just fine, as long as they get the proper exercise and training.
Sounds to me like the OP is committed to their dog, so I don't think that it is appropriate to question their suitability to being GSD owners.
I second getting some outside input from an outside trainer - all the aversives that you have tried have not worked, and the pup should have outgrown these behaviours by now, so I'm with those that say that you are inadvertently rewarding him for trying to engage with you in this manner. Your timing for corrections may be off, or a number of other small, subtle behaviours and reactions from you could be what he is looking for, and thus as others have said, have been re-enforcing the behaviour instead of discouraging it.
Having someone else observe the interaction may be able to pick up on these dynamics and show you how to stop the cycle.
Personally I think you dog needs more mental stimulation - I can bike 10 km or hike throught the bush for 3 hours with my dog, and it doesn't seem to make much difference in his energy level, but a 15 - 20 minute session of obedience will get him pretty tired out.