How would he possibly establish such channels? Their domain is seized along with their wallets...there would be no way for him to verify their identity.
You can't have it both ways (well, you'll certainly try). Either Bestmixer can contact Hhampuz to arrange a refund or some other action, or they can't and Hhampuz is acting on a previously-arranged plan or his own judgement by sending the coins somewhere safe, just to mention a couple of many possibilities why the funds may have moved. You have to employ a bunch of fairly stupid assumptions to make it look like a scam, starting with the highly improbable premise that Hhampuz couldn't be trusted with half a coin after he was handling many times that just for this campaign alone.