It is a major risk. It's pretty easy to tell one artists work from another, especially when it's the quality of PG's renders as his use of DoF and lighting is rather masterful.
This is actually not a valid concern, from a legal perspective. Yes, most good artists do have distinctive and unique styles, even when using a program like Daz, but there's no way that a prosecutor could prove in a court of law that Artist X created a specific piece of art, based on the appearance of the art. Showing similarities in style between two pieces of art cannot conclusively demonstrate authorship.
Metadata on the images would probably be sufficient, if the image used by the prosecutor were found on the defendant's computer, and not a copy downloaded from anywhere else, which brings us to your other point.
If they seized his computer (which they would if they were attempting to bring charges and would need no proof to do so other than reasonable suspicion) then proving it would be child's play for even a novice computer tech.
This would be the real concern. Of course, they would have to get a warrant to seize the computer, which means that they would have to show a judge that they had sufficient grounds to suspect a crime had been committed. It would be up to the judge to determine whether or not their grounds were sufficient, and also to decide the scope of their warrant.
So it would really be a question first of what data Mega forwarded to the New Zealand Police, second of whether or not the NZP actually forwarded that data to the authorities where PaleGrass lives, and third of whether or not that data (or any additional data subsequently gathered by the Irish police) would satisfy a judge's requirements to issue a warrant.