He already has that defense simply because this is one of the most publicized stories in recent years. It's only a question of whether some judge (military judges at this point) will allow such claims to weaken overwhelming and irrefutable evidence.
You entirely dismiss the bureaucrats (and presidents) great desire to cover their own asses. The longer this investigation is within the control of executive branch agencies, the longer they have to cover their past mistakes, destroy evidence and generally shape things to minimize the blame that can be put on the PC infestation of all government agencies.
Many government agencies are guilty of extreme poor judgment here, going back years, and they will do anything they can to avoid any culpability at all. This investigation should not be left in the hands of people such as General Casey and Obama.
And if you have some naive notion that these high ranking officials and bureaucracies in general won't go to many lengths to cover their own asses, then, well, you don't even belong in this discussion, or maybe you're covering for them for some reason.
The sooner entities outside the executive branch begin investigating this, the better. They might already have called in Sandy Berger to assist the executive branch agencies 'internal investigations'.
Yes, bureaucrats will cover their butts.
All I am saying is, bring him to trial and convict his murderous jihadist ass before interfering. I have yet to see a congressional investigation that accomplished anything.