The first part of your theme seems possible........... but likely NOT complete b/c in my thinking it would take considerable time for 750k btc to be absorbed completely into the market. The second alternative seems even less possible b/c it seems very unlikely (though possible) that the stealing would have ONLY been by one thief.
Let's say there is a thief and his name is not Mark. Either the coins were stolen slowly or in very short period of time.
what would you do if you were the thief?
If you stole the coins slowly over time, you would have no idea when you will be discovered. also you know that once the hack is discovered the price will tank, so the best move would be to sell them as soon as you get them.
If you stole 750k coins very quickly in 2011, it wouldn't be worth much at that time, you know the price will tank when discovered and it would reasonable to believe your hack could possibly kill bitcoin forever, you best bet would be to offload it in bulk to large buyers in the underground world.
In the unlikely event 750k coins were stolen recently, that would make it the biggest heist in history, you certainly can't think you freely can sell it on any exchange, especially since mtgox knows the addresses and can follow the blockchain. Congrats you have the same problem as satoshi.
My conclusion there is no thief with 100k's of coins.