FWIT: I would drop back 15 yards and start fresh with a good "catch". Erase everything that you think you know about what's been going on because you have been comparing apples and oranges. Now that you have your Vet's attention and he shows such willingness to learn from others (you are sssooo lucky he counts Dr. Couto as a friend!), this is an opportunity to do it right.............the second time. Besides, at this point, it doesn't matter what the test results were last year or the year before that - you wouldn't treat Max for an infection that he may or may not have had in 2006, so why muddy the waters with it now?
Have patience. As you said, Max has been healthy up to this point. Let's hope that his injury and the meds have his system a little out of whack and with time and TLC, he'll be back to his old self. In the meantime, relax, breathe deeply and formulate a plan that makes sense to you. Please keep us updated. Sending prayers and hugs for both of you.