In my early days of using my 2012 TI, I experienced a failure of my rudder line >inside< the hull, and a combination of wind conditions and my inexperience with the TI resulted in my being swept 7 miles out to sea before being rescued.
In a dose of overkill, I have added extending arms to the rudder, with the port external line being rigged with 2 to 1 leverage, leading to a cleat beside my seat. On the starboard side, the extension is connected to a line which contains some bungy cord, and leads to another cleat to the right of my seat. In the (highly unlikely) repeat of failure of the internal lines (which I believe was caused by incorrect leading of the line against a scupper tube), or slippage at the rudder, I can tension the starboard side, and then steer by applying pressure on the port line. The photo shows Mk i of the extenders, which have now been replaced with stronger pieces.
As regards the rudder lines themselves, I suggest you add a washer to that screw, and wrap the rudder line a full turn around the screw before tighteninng. I doubt you will get any slippage with this arrangement.
2012 Tandem Island "SIC EM"www.scenefromabove.com.au