After reading this issue, I've come to the conclusion that the Superman Blue/Red theory is correct, but that we've all been wrong about which Superman is which.
A few weeks back, I was re-reading Superman Forever and noticed this:
superman forever (kebbin) #12.jpg
Superman Red went into SPACE while Superman Blue went into the center of the Earth. At the beginning of Superman Forever, we see the restored Superman (or so we think) plummet back to Earth, landing where his ship did years before.
If the Red/Blue theory is indeed true, it could very well be that Superman Red was the one who was "restored" and continued on as Superman. We've assumed it was Red who became New 52 Superman because of his temperament and that Blue was the pre-Flashpoint Superman who lacked drive and direction before Flashpoint. The fact that both were incomplete could still explain the inadequacies in both Supermens' behavior.
This would also explain the blue energy seen around the mystery Clark in this issue and the fact that he seems to be quite milquetoast and goofy, traits that could be attributed to Blue.
This would mean that the cover to Superman #19 is a red herring (no pun intended), showing red energy around New 52 Superman and blue around pre-Flashpoint Superman.
If Mystery Clark isn't Red or Blue, I don't think he's pre-Flashpoint Henshaw. His rant at the end of the issue reminded me of either a confused and overwhelmed Superman Blue now realizing who he is and what he's missed/"had taken from him," OR
Superboy Prime.
I got that whiny, "you took everything away from me!" vibe that Prime had. I don't want Prime to be used as a villain yet again, but he's definitely one of the possibilities at this point.