How could they be sold on Gox in 2011? Wouldn't the thief(s) be identifiable through blockchain forensics and have their trading accounts closed? It just doesn't make much sense. IMO, the 2011 downtrend looked more like people just being worried that they invested in hot air as bitcoin wasn't useful for much back then.
Didn't need to be sold on Gox or instantly. One theory could be that Mark sold the coins back then for cash knowing that more bit coins were com in in all the time. The cash was maybe to be used legitimately for improving the business or the general Bitcoin eco system but as the price bubbled again he couldn't buy all the coins back.
As beens said we won't know the full story for a while if ever but with the evidence we've been shown nothing should be discounted.