Robert Riddell was born on 31 October 1891 at 19 Lamb Street in Hamilton to Robert and Catherine (McCormick) Riddell. In 1901 he lived with the family at 5 Craig Row in Auchentibber, a small hamlet near Blantyre. By 1911 he was working as a coal miner and lived with his family in Kirkland Rows in Burnbank.
On 31 December 1917 Robert married Jeanie Crichton at the Manse in Burnbank. He was 26 years old at the time of their marriage, lived at Dykehead Rows and worked as a coal miner. He rented a house from William Boyd, a tailor, in Dykehead Rows for 6 pounds, 10 shillings in 1920.
That is the last piece of definitive information I can find for Robert Riddell. I am fairly sure he did not die in Scotland and think he emigrated some time after 1920.
1901 Scotland Census Parish: Blantyre; ED: 1; Page: 10; Line: 13; Roll: CSSCT1901_245
1911 Scotland Census, 02/04/1911 Riddell, Robert (Census 1911 647/00 036/00 011)
1920 Scotland Valuation Rolls, 1920 Riddell, Robert (Valuation Rolls VR107 /347 / 202)
Scotland, Select Births and Baptisms, 1564-1950, 1891 Riddell, Robert
Scotland, Statutory Registrations, 1855-2013, 1891 Riddell, Robert (Statutory Births 647/00 1101)
Scotland, Statutory Registrations, 1855-2013, 1918 Riddell, Robert - Crichton, Jeanie (Statutory Marriages 647/00 0003)