I'm not sure I follow your logic, cote - are you saying that people shouldn't be prosecuted for viewing images of child abuse unless the images are date stamped, the child located, and a birth certificate made available? Because that's the logical consequence (as far as I can see it) of your argument.
You seem to be very keen to find ways in which the inexcusable can be excused - it may have been a 16 year old that looks like a 12 year old. Therefore it's ok to view images of child abuse, and therefore it's not pedophilia... For me, this is taking plausible deniability too far.
I think I'm getting confused about if you're arguing about this particular case (which follows the script predicted by Queen Tilly) or if you are arguing that there may be theoretical cases where someone may be charged for doing something that could be argued to be innocuous by some people.
I'd find it constructive to know which of these lines your taking:
a) Gresham was correct, it was silly that his friend was prosecuted given what Gresham stated was the case
b) Despite the further knowledge of the case we now have, Gresham was still correct, his friend shouldn't have been prosecuted
c) His friend should have been prosecuted, but given guidelines on how to avoid downloading and distributing images of child abuse
d) There is some theoretical possibility that someone could possibly fall foul of the law by mistake.
The thing is, that in most western democracies, the judicial system interprets the law, so if you are arguing d), I'd say it's a hollow argument unless you've evidence of someone who has been prosecuted under these circumstances.
My child could throw a snowball, which bounces off a tree, hits a car windscreen, causing the driver to swerve into a pedestrian, and be charged with manslaughter. But let's face it, despite it being a theoretically possible outcome - it just wouldn't fly in the actual courts...