"Here's the right turn he would have had to make, Streetview image from only about six weeks before his journey. There's a big sign warning of major roadworks at Ardlui."
I hadn't noticed the sign at Crianlarich indicating major roadworks at Ardlui (from March 2010)
Further down the A82 there is another roadworks sign 1 mile north of Ardlui (from April 2010) showing "delays possible until July 2010"
Heres the actual roadworks, traffic light controlled and single lane traffic.
Assuming the roadworks were ongoing from March 2010 to July 2010 then on 5th May 2010 the roadworks were still there.
This means that either Gilroy did not notice the roadworks sign at Crianlarich and continued on the A82 thinking it was a quicker route to get to Edinburgh (if we are expected to believe this explanation) or Gilroy did see the roadworks sign and even though he knew there would be roadwork delays on the A82 it was a vital part of his plan to take the A82 from Crianlarich.