Destroyer of Islands
Deactivating a knight causes a knight to "do stuff". If it's not movement or going towards the defense of Catan, then that can be used to chase the robber away. The point is the knight's presence alone isn't enough to chase the robber away.
Correct. The robber can be placed on a hex where there is a knight. Then on the turn of the player who owns the knight, that player can deactivate the knight to move the robber somewhere else (and steal, as usual). The player cannot, however, activate the knight and then immediately deactivate it to move the robber, since a knight can never be deactivated on the same turn it was activated on.