Thermostats rarely get stuck open and unless you have fitted a failsafe one will always fail closed and lead to overheating.
The by pass vavle should have been fitted as a recall and VW would have notified all the owners of the affected cars. Open the bonnet and look just infront of where the coolant hoses enter the car for the matrix, if you see a valve then you have it.
A best bet is to take the rad off the car and invert it, then reverse flush it as if you have a blockage you're gonna flush more crap into it leaving it on the car.
