John Mcdiarmid is 88 when he appears in front of Lord Napier in June 1883. Tracing someone with a surname like "Mcdiarmid" presents a few challenges. There are quite a few possible permutations in the spelling of that name, certainly on the census forms. What this piece of research also showed was the inconsistency in ages. Are the death certificates any more reliable than the census forms?
However, reading the evidence yields a few clues, the most important one being that he spent a great deal of his lifetime around "Strain" (Strond?) in South Harris, and the census of 1841 does pick him up there.
In 1841, John Mcdearmid is in Strond, Harris, aged 40. He lives with his wife Mary (35), who we will next encounter 40 years later. Their family consists of Marrion (20), Donald (15), Marrion (10), Christina (8), Mary (5) and Ann (2).
In 1871, a John Mcdermid is quoted as a lodger in Strond, aged 70, living with a family of Shaws. I have my doubts about the 1871 census entry, because he is shown as being retired, but not living with his wife. One of the death certificates for a John Macdermid in Harris shows a widower who passed away in Strond on 6 January 1885, which bears out my suspicions. We can therefore discount this entry.
A decade later, just before he gives evidence to the Napier Commission, John Mcdearmid is once more shown with his wife Mary in Island of Scalpay, as a retired missionary.
Direcleit has filled in the gaps of other census years, see his comment below.
John Macdermid passed away on 5 May 1884 at the age of 88, having suffered for hemiplegia for one month, and cerebral softening for 2 years. He is listed as being the wife of Mary Ferguson, and his parents were Angus Macdermid and Mary Macleod. His death was reported to the registrar by his son Donald.