Hi Umu
OK, I see what you mean. As you say, it's difficult to see from the drawing what tightening up the top nut would achieve. However, (when you next get back there) giving the collar a 1/4 turn is easy enough and might have a good outcome, otherwise it sounds like a rudder-out job to check the bearing surfaces. If that's the case, I do remember one of the guys on here suggesting strapping an empty dumpy-bag to the sheet winches to catch an accidentally dropped rudder. Top Tip, that one...
cheers