Caz
You have hit the nail on the head regarding pre-planning I have explained in the past that other waypoint coords had their apparent accuracy from conversion from grid to lat/long of convenient line intersections and that it was waypoint A that did not fit this category and was right for that LZ something even RACAL did not recognise in their report and none of the venerable aviators here picked up.
The lat/long for waypoint A was rounded just a little (didn't take much) for convenience as one would when actually being there on a previous occasion and noting a convenient set of lat/long coords with their own nav system (as opposed to relying on map accuracy).
Apart from waypoint A being a marker for the LZ, it was also the sensible point to aim for if intending to swing around the back of the lighthouse (these beasts have a significanty turning circle at cruising speed).
035 mag back then was the optimum approach for that LZ (long axis of LZ and lined up with a prominent feature that a lighthouse keeper told me they always crossed) such that if they either changed their mind about landing there (wanting to wave-off) or otherwise were just going to swing around the lighthouse they just started an nice banked turn (anything from 30 to 60 degrees) on crossing the shoreline at waypoint A and this kept them clear of the masts and the hillside surely a well known point for crews dropping squaddies off on exercise from NI.
Just one thing, you wrote <<... but it could explain the "Misplot" and why the Crew did not turn immediately when they switched waypoints. >> - it does not explain at all why they did not turn immediately when they switched waypoints waypoint A would still have been of use (as they still had some way to go to the location/shoreline) in the absence of any other reference system and you well know my theory on that.
By the way, all, whether you like it or not, a piece of equipment that could have been used for a local navigation exercise/demonstration has been recognised as having been fitted to some HC2 Chinooks back then some evidence (documentary and hearsay) suggests that it may have been fitted to ZD576; forget the waffle, after all these years should we not know whether it was fitted or not with absolute certainty? Doesn't matter what your feelings about the causes of this crash, was this kit fitted or not? How can anyone with sincere interest in this disaster possibly ignore this issue? - even mobile 'phones got consideration.