Yes, the one who died in Mortlach in 1871 was the son of Helen Stewart - his death doesn't state his father's given name. So he can be eliminated from your enquiries.
If he was listed as 55 in the 1841 census, which was taken on 7 June, and assuming that his age is accurate, he could have been born between 8 June 1781 and 7 June 1786.