That is a bit of an odd design. There is a spring on the lever to push the tang away from the castle nut but it appears the factory didn't even trust it to work as they put a locking screw on the end of the lever just to make sure. I would be tempted to take it off and just use two wrenches instead. That would probably be faster as well.
Someone I consider a master woodworker once told me that a master woodworker is not someone who never makes mistakes. He is someone who is able to cover them up so that no one can tell.