if you are getting any erratic readings from the wheel sensor (pot) then that is probably your problem. You should get a clean sweep on your meter when checking it from 0 to it's limit which should be 5k.
If you are going out of range on one end of that sweep that would explain why you are loosing it when turned fully in the one direction.
As for wire guidance the only part it interacts with the steering is when wire guidance is turned on it looks for the steer wheel's position and if not 'centered' it forces the system to turn the steering to 'center' when on the wire and initialized. When off the wire it should allow steering to operate normally.
Putting the new pot that you ordered on should fix the issue.

The proximity sensor on the steer wheel is the pot mounted there sensing the position of the wheel position. Some units have a circuit board mounted on top with position sensors but in this case it sounds like it is just a pot.

There is a proceedure for setting the steering center and straight travel but you will need the service manual to set this. It is rather involved and takes 2 people at one point.

One quick way to see if the wheel is straight though is to remove the cover on the EPS steering unit and check LED's 1 and 2. When the drive wheel is straight they both should be lit.
One part of the MDU pot setting check says to check wires 16-17 and 17-18 and both readings should be equal within 100ohms to be considered as centered.