In this hypothetical, who is doing the altering?
Was the Cary PD able to alter the history so completely that it fooled the FBI experts, or are the FBI agents in on it too?
Since the defense is claiming the Cary PD was inept with electronic evidence, it's hard to make a credible arguments for the former. And, it's even harder to come up with a motive to cover the latter.
I don't believe it was tampered with, but the possibility is certainly there. No matter what anyone tells you, I feel confident that if I was given enough time and complete physical access to this computer I myself could fabricate the evidence without anybody later detecting what I had done. It would not be easy and is unlikely to have happened, but possible with enough time and energy put into.