Fletcher Alias McDonald in Taynuilt

My great grandfather's brother, John FLETCHER, has long been a mystery to us. Wondering if anyone can see something that we are missing??

John was the son of Alexander FLETCHER, a shepherd, and Catherine MCPHAIL. This couple had 10 children - John, Alexander, Angus, Ronald, Colin, Margaret, Catherine, Mary, Lily and Jane, all born at Achadeshenaig, Glen Aros, Mull, Argyll in the mid 1800s. Today, we found John's birth on SP but it only poses more questions. He was born on October 25th, 1858 but strangely enough it looks like he was originally called/christened (maybe even registered) as Archibald and then later named John which was registered on November 6th, 1858. 

He worked at the Salen Hotel in the 1870s and 1880s, then disappears. We have very few clues, at least none that have helped us so far. The story goes that he joined the military and at some point deserted and then lived the rest of his days in/near Taynuilt under the assumed name of John MCDONALD. A nephew took care of his affairs after his death and it was he who supplied this information. John MCDONALD is such a common name and so many villages and parishes border Taynuilt, and the lack of a death year all make this a difficult task. He likely died between 1930 and 1950, so I wouldn't be surprised if some old native of Taynuilt might recall him.

John FLETCHER (alias MCDONALD) may have been married. The clue that hints at that is a circa 1900 photograph of my grandfather, Alexander FLETCHER (1890 - 1950), and all his siblings plus one extra child. This boy was only known to be called John and died not long after this. He may have been born around 1890, from how he looks in the photo. 

As you can see, we have little more than tantalizing clues. My father has been going through John FLETCHERs and John MCDONALDs on SP but there are too many of them! Thanks for any ideas.