Is it possible to have the wallet.dat and yet have no access to the coins? Why haven't they been tumbled yet?

What is the rush? Transactions are irreversible. In prior hacks attackers have waited days or even weeks before they starting obfuscating the trail.

Why wouldn't Bitstamp just move the coins to a secure wallet themselves if they have the private keys/wallet.dat along with the hacker

Huh

The assumption is the address containing the stolen coins is not controlled by BitStamp but is instead under the control of the hacker.

What do you mean assumption? I thought it was news