Historian as expert witness

Here's a story of an ongoing trial involving a man who is thought to have been a Nazi guard at a death camp:Historian doubts Demjanjuk's wartime accountTwo questions:1) in the story they talk about a military historian who testifies against the defendant.  How often have you seen historians provide testimony like this at trial?  It's interesting that the man's fate may rest on the accuracy of the historian's knowledge.2) I am not sure about the legal effect of the Nuremburg trials, but it was surely the case that the more senior members of the Nazi party (and I'm guessing those in charge of the death camps) were tried, imprisoned, and executed soon after the war.  As far as I know, the lowest level of the Nazis were let off the hook.  Yet this present trial involves a man who is claimed to have been a prison guard.  So at what point in history did attention turn back toward prosecuting lower-level war criminals?  Was this a reversal of what was decided at Nuremburg?