JP,
The yachtsman could see the lighthouse, so it is very likely that the crew of the chinook could too.
I have flown a very similar route too, in pre TANS Wessex days, and would bet a pound to a penny that the intention was never to overfly the lighthouse, or any of the high ground. I would have positively ID'd the waypoint, or mull, and then turned left to 'handrail' the west coast to the mouth of the Great Glen.
Now. Why they did not do that will never be known to a balance of probability, let alone with no doubt whatsoever.
You are postulating as FACT that they saw the high ground, changed waypoint, and then deliberately flew on to their deaths.
Which of our two scenarios has the ring of truth, then?