I don't think dutch roll had anything to do what we are talking about. I think rudder movements caused the AA 587 crash and I will bet it had nothing to do with FO rudder movements. Yes, the rudders moved but what moved them? Where does the FDR sense the movement? I think it is the rudder actuator so what caused the actuator to move?
My first thought would be yaw damper, not a totally stupid pilot stomping on his rudders. I think both pilots were along for the ride because the captain would have prevented FO oscillations as all of us would. The yaw damper malfunction has happened before as in my previous post.
The FO didn't cause it in my opinion. NTSB has political pressure to lean to what works their way.