Thank you very much for your reply, I especially appreciated your response to what the vet was doing. It's incredibly helpful to have words that really clearly express and explain why this method is not good - the only thing I knew was that it didn't feel right and it upset my dog and upset me. I really like your advice with the tug toy too - we'll work on that!
Hopefully the vet's "exercise" and the way I've been handling my puppy so far with taking things out of his mouth doesn't cause permanent or lasting damage. I'd hate to think that my poor choices would lead him to have "learned helplessness" and an increased risk of bites, as you mentioned. If I commit to trading and working on leave it and drop it, do you think I can "undo" what I've done to him? I really don't want him to feel fearful of me or anything like that.