(a) Don't know, but it's beside the point. The legal definition is the correct definition which applies in the particular state or territory of Australia. Taking another meaning is actually taking the wrong definition.
(b) The NSW definition has been in since 2014, when the Road Rules were enacted. It was probably there before that, but I don't know what the previous regulations' name was to check . In Queensland, where the same definition is used, it has been in place since at least 12 October 2009 (which is as far back as the current regulations go). So, possibly before that as well.
(c) You and I can't. The government can (and has). The government has the power to define u-turn to be whatever they want it to be.
(d) Purposive interpretation means that the courts look at what they consider to be the purpose the law seeks to achieve, and adopt that interpretation. This is as opposed to a strict (or sometimes called 'black letter') interpretation. Therefore, in a hypothetical example: If it was accepted that the strict definition of turn required a single continuous movement but the magistrate decided the purpose of the road rule was to make it unlawful to turn around in that position regardless of how many corrections were needed, then the court would ignore the strict definition of turn in favour of the meaning which best accomplishes the purpose intended by legislation as the court believes it to be.
Personally, I have concerns with purposive interpretation and the lack of certainty it gives - not to mention encouraging sloppy drafting; but it is gaining acceptance throughout the court system.