I know it won't convince the most stubborn supporters of the theory, but it will shut down the less zealous ones.
Well, even a corpse wouldn't be conclusive evidence that Justin is
still dead. He could've died, but remained on Earth the way Corpsetaker did, and how Kemmler may have done. Dead but not gone. Then Justin's soul could have found a way to resurrect itself by taking control of a new body, or-- well, who knows what? Corpsetaker's method of resurrecting herself may not be the only method available to a soul still on Earth.
For one thing, the Sidhe seem to be able to see and interact with souls just as they would with embodied mortals. They can even make deals with those mortals, as demonstrated by Harry and Lea at Harry's grave in
Ghost Story. So why couldn't Justin make a bargain with a fae for the fae to find him a living body that he would be able to take control of or possess? Or for the fae to arrange for such a body to turn up somehow, free of the soul of the original owner? Other details could be hammered out as well, like the gender, magical potential and so on.
Justin would have to offer something fairly important for this deal, most likely, which could take it out of his price range, but it is possible in theory.
All of that said, I don't think Justin's alive, myself. I think he's probably dead, since Jim has told us so repeatedly. I'm just playing devil's advocate.