This is a pretty horrific story. You’ve been treated rather badly by the people who have worked on the car up to now. Starting from where you are, after the work already done on the car, you can, assuming the work to have been done correctly, discount the gearbox itself and the clutch. How this could be an electrical fault I cannot imagine! That leaves the gear selector linkage, which could be out of adjustment. What does the gearstick feel like, when you attempt to select 6th gear?
How long have you had the car and what sort of warranty did you get with it? You would have been better advised to take this problem straight to VW when it first occurred, as it shouldn’t have happened with such a low mileage car. Assuming that you have had it serviced properly, they would probably have been sympathetic. As it is, a lot of things have been done to the car which muddy the water.