I am familiar with the area and have flown close to the route many times at low level VFR.The theory by Roy Conyers Nesbitt is in my view the most likely. A transit to overhead Thurso is safest by following the coast and turning inland at Lybster.The Highland airways fleet of DH Dragons and Rapides used the coastal route and could transit under very low cloud by being just offshore..although Capt Fresson did have a near miss with the top masts of a battleship just off Golspie.