Well, I'll throw my two cents out there just for fun. I probably won't say anything you don't already know, especially if you've had dogs before Mr. Buddy!
I'm going to interpret the "leadership"/"parenting" vs. "dominance" issue as a question of semantics. Sounds like everyone is really in agreement about the basic issue: like all young kids (and furkids), Buddy needs boundries and discipline from his "parents." That word "discipline" can be an easily misunderstood word with definite negative conotations, since it is unfortunately often achieved through punishment, but we wouldn't realistically raise our own children without a clear set of expectations and consequences and no dog or wolf would raise her pups without the same. Like Bill said, the concept of dominance is a bit off the mark. Turid Rugaas writes beautifully about the filial love and devotion that truly rules the pack - I prefer her explanation.
I was really surprised to read that your behaviourist suggested less exercise. Especially given Buddy's mix of breeds, that seems very counterintuitive. Grossly generalizing here, he comes from a working background and is bred to be extremely smart and speedy. Could be that he's looking for sufficient outlets for his great mental and physical energy. Simply walking (even outside the house) might not even be enough - how about biking with him, or getting involved in some agility training, or even sheep herding (not sure where you are located but there may be a club)? Even if there are no formal agility courses near you, there are TONS of online resources and literature to get you started on a DIY version in your own backyard! Sounds like he is a really clever guy (having read back through some of your previous forum posts) so he'd probably love the challenge.
Regarding the walks, once some of his built-up energy is drained, you may find that the pulling, jumping and biting at the leash lessen. Also as Bill said, you can help him drain energy and learn self control (which all puppies need to learn) by establishing calming rituals before, during and after the walks - this could look like sitting to have his leash put on rather than jumping or pacing, sitting at the door to wait to be invited on the walk rather than charging out to meet the world, and sitting patiently after returning home to be invited into the house and rewarded with a big drink of water. All these little rituals will help him to focus on the task of walking/jogging purposefully (this turns the walk into mental exercise as well as physical) and a more focused dog is much more biddable.
Best of luck - let us know how your work with Buddy is progressing!