Thankyou for that illuminating and toe - curling tale!
Not long ago I had a conversation with a now-retired 757 skipper and I asked him the same questions. Essentially he said that the pilots should decide when to turn on to the stand as it isn't reasonable or practical for a marshaller to be able to make that judgement as different aircraft have different turning characteristics. He added that generally speaking the turn signal is ignored by the majority of pilots anyway! I remember a similar conversation with a 737 pilot many years ago; his angle was something like, 'we just need you to tell us when to stop'. I agree with both sentiments.