The truth is that Obama wanted to follow through on a promise to close gitmo and so he released the five Taliban leaders. He figured that he could use Bergdahl as cover to add legitimacy.
Whoever managed the Bergdahl trade....probably never asked the Army for their report on the guy or avoided discussing this side of the deal entirely. He likely believed everything the family said and just figured it’d all work out.
Personally, for the defense lawyer on this case...I think the only way to play this is to say Bergdahl was ‘nuts’ as he graduated high school....went off to Coast Guard boot-camp....failed that boot-camp....and then got the Army to waiver him for their boot-camp. The Army will provide more than enough witnesses to say he was nuts from day one on...just use them and confess to being a nutcase. If you go any other route...you lose the case and Bergdahl sits in a prison for the rest of his life.