I don't know how the 3.5L pulley is constructed, but if it's like the 3.0L, there's a key in the crank nose that receives the sprocket and the pulley (vibration damper). As you said, I think the pulley on the 3.5L is located with a pin. Some torque is transferred through the pin and some through the bolt-provided friction - whether there's also a keyway in the pulley, I don't know. In your case, what sounds to have happened is the rubber between the pulley hub and the outer ring has spun - but I don't see how that would mess up the crank or the sprocket.

I would think that if the sprocket had spun on the crank, you would have trashed all of the valves, since the 3.5L is an interference engine.

It's possible that the bolt loosened enough for the pulley to back out just far enough for the pin come out of the locating hole and spin, leaving the bolt fairly tight - or it's possible that the pin sheared, allowing the pulley to spin against bolt friction.

In any event, you'll need to take it down to find out what happened.

Frank