I have to report that the LM tune in Fr Gordon`s manuscript is not known to our Banchory fiddlers. The former leader of the Strathspey and Reel has hunted without success in his stacks of tunes.
I`ve also had a look in the Scottish Music Index - Scottish Fiddle Tunes of the 18th and 19th Century. It`s similar to the Hymn Tune Index in providing a search by incipit, or rather by the first two bars. Indeed it`s better than HTI in providing a search by the second bar as well as the first. But there are also difficulties, like that only the beat notes are used, not the intermediates.
The SMI let me search the printed output of William Marshall - over 100 tunes in four collections - but nothing resembled your tune for Iam lucis orto sidere. Well that`s if I am understanding the notation properly.
I wonder if you have tried Ian Russell and the fiddle experts at his Elphinstone Institute, Shelagh.