The fault code of "engine start blocked by immobiliser" will be present if it is the immo causing it. If this fault code is no present then it is not the immo.
Why not the 12v sump? it is made of a material that doesn't crack when you look at it wrong like the 24v ones. With how low your car is, a few millimetres isn't going to save anything.
This is possible to fix with a few quids worth of high wattage resistors and a decent soldering iron. Its common knowledge that a fan motor with more than 2 big wires going to it has a resistor in it for speed control. The VW dealer that told you otherwise should be shot!
This is MOT tester speak for "You need new top mounts mate" And is a very grey area on Golf top mounts, as they are designed to work with the weight of the car on them, so guess what happens when tested while being jacked up?
You just said the fault code is an immobliser code? What are the exact fault codes stored? Immo box is right up behind the dash (read extremely hard to get to)