MtheGM -- I have to say that I'll take Norman Buchan's word for it any day.

In '101 Scottish Songs' (aka The Little Red Song Book), Norman attributes the song to John McEvoy and the notes at the back say "When the Stone of Destiny was removed on Christmas Day, 1951, from Westminster Abbey it seemed for a time as if all Scotland was busy writing songs about the incident.Some of these songs were pulled together in a small publication called 'Sangs o the Stane'(Scottish Secretariat). Of them all it is 'The Wee Magic Stane', written by Johnnie McEvoy - sic! - which has passed most quickly into popular currency."
101 was published in 1962.

Quite apart from anything else, Norman and Morris Blythman were very close friends and had stood as best men for each other when they both married, so there is no chance IMHO that Norman would either have been unaware of Morris' authorship had he actually written the song, or would have wilfully attributed the authorship elsewhere.
In my experience, Norman was always very concerned that people were properly recognised for their contribution.