Here's my take:
This accounts for where he was an why. It also accounts for the father insisting it was Bush's fault. The FBI interview was the kiss of death.
My take is a little different. He's hired by Moore to help with his schlockumentary, goes over there under the cover of "looking for work" and mingles with the populace. Gets some footage at the prison with willing military. Continues on, meets up with some of the bad guys, who view him as useful, but perhaps not in the way Berg thought. Note the beheading footage where he seems completely calm and unconcerned that he's their "prisoner." No, the beheading was not part of the deal he signed on for--but these people have no loyalty except to themselves.
Most sources said he was broke, and hadn't found work. However, he spoke of plans to visit Turkey for "sailing," among other things. He had money, then, after all. We've all wondered where it came from--my guess? When he got the prison footage and shipped it stateside, Moore gave him a bonus. Moore must have thought he'd struck gold.
No one--to my knowledge--has ever found proof that he "supported President Bush and the war" beyond his father's word. But his father also stated emphatically that his son was the best friends the terrorists had.
You are onto something. As far as I recall, there was never really any plausible explanation for Berg being picked up. With his history (and dad's) here in the U.S., our gov. was checking up on his activities in Iraq.
Daddy's lawsuit insisting Nick be released into Iraq's general population was the tongue down the throat.
That works for me.