Yes, you are dropping the ball when you try to rebroadcast the dragEnter. You are creating a new DragEvent and sending it, but the newly dispatched event does not carry the key bit of information from the original event: the object being dragged.
So when the Mediator tries:
private function onArchiveDragEnter( event:DragEvent ):void
var obj:IUIComponent = IUIComponent(event.currentTarget);
'obj' will be null, and therefore the accept will fail, causing the DragDrop event not to be sent.