How can hackers cash out stolen bitcoin?
Interesting points Paper, here some of my thoughts...

As you said, on the underground (i.e. TOR) there are forums where thieves are selling dirty coins, with a favorable exchange rate (1 clean coin for 2 dirty coins for example). But to avoid scam I suppose they will use some kind of anonymous escrow service to perform the exchange, to guarantee that both parties are willing to complete the transaction. 

Mixing service are also risky since they only guarantee anonymity if there are actually other user's coins to mix with. I remember some time ago, when a thief sent a huge amount of stolen coins at once to a mixing service, the returning coins were practically the same since on the mixing service there wasn't other users mixing such big amounts.

I don't get the point about "his network of Bitcoin address" what you mean? You can generate thousands of addresses and send coins back and forth, but this will actually just obfuscate (you can always track from start address to destination). The only way to really mix them, is to mix with other user's coins.

Furthermore many stolen coins are probably used to purchase illegal items from hidden markets  (drugs, weapons, pretty much anything). So they are actually "enjoying" them without the need to exchange them in "real" currencies.

And last but not least, in person exchange (for example thorough localbitcoins.com or another local directory for Bitcoin buyer and sellers)

Cheers