Interesting, but seems like a stretch.
It does seem suspicious that such a huge percentage of cold storage could be drained without notice, there could be some merit to that line of inquiry, but I don't think it is at the hidden behest of the US Government.
No, that's not a stretch at all. This is among one of the most believable theories along with the theory of other powers interested in harming BTC have used Karpeles as a tool to execute this or blackmailed him to do this.
A stretch are theories which involve incompetence not realizing there are not enough BTC in cold storage + hot storage to back up the BTC funds in the MtGox server database for so long. This would reduce Karpeles and the MtGox team's IQ to less of fruit.
A stretch are theories that involve a scam artists managed to steal the bitcoins or malleability being the reason, which would show in many cloned transactions going out from MtGox easily to be followed via the blockchain if Karpeles released the books to some of the trusted experts in the community.
There is a reason why he does not so, and why he does not beg the trusted experts in the community, like antonopoulos, gmaxwell and the like to visit him and check his books.
A stretch is to believe that even if the 750k were gone, the exchange had no means to get those easily back via arbitrage it could have done and use a bot to trade on their exchange at no fees, allowing them to profit from the smallest price differences in buy/sell. As well as many other countless ways to manipulate using the exchange powers when you are desperate to get out of fractional reserve operation.
This leaves us only with a few options, all of which involve there having been some intentional attempt to harm BTC, with Karpeles either willingly participated in or having been blackmailed so badly, he saw no other way out than to succumb.
This is no excuse to Karpeles, knowing how this would mean the destruction of thousands of lives out there, however i am not sure myself how i would react if some men well dressed in black coats (NSA/FBI/CIA) visited me and made it clear that it is either their way or the highway...
edit: Adding to the above. It is my true belief (i could be wrong) that Karpeles is in real danger to fear of his life if the above is anywhere near accurate. Because if the above is close enough to the truth, they will want to get rid of all tracks, especially Karpeles and the gox records/books.
The only way to get rid of this risk, would be to release all information publicly as fast as possible. Or at least give the information to someone trusted in secret who would release it in such an event, and make it known to those powers.