What is Ethel Euretta "Retta" McMartin's story and who were her parents?

Ethel Euretta Retta McMartin is my great-grandmother and wife of Benjamin Laybourne. She is reported in 1921 Saskatchewan Census as Retta McMartin where she is listed as a 24 year old (born approx. 1897) having already had her eldest child in 1915 at the age of 18. Married to Benjamin Laybourne. Died in Nelson in 1961. Grandchildren recall and report the particulars as to her name and date of death. Unable to find who her parents were. She goes on to have three more children, my grandmother (Freda) being the last in 1933.

Three options of who her parents were?

Option 1: The Cameron/Nipissing, Ontario "Ettle"

In 1921 Ontario Census - "Ettle" McMartin listed as a 21 year old (born approx. 1900) at line 43 and living at home with widowed mother Josephine and siblings (William Gates being biological father of this McMartin who passed in 1916) near Cameron, Ontario.

Unlikely my great grandmother is this McMartin, not only as a result of the double census entry (she is listed in the same year in Saskatchewan married to Benjamin), but accepting this record necessarily requires two additional unreported children being missed in the 1921 Cameron, Ontario Census. Martin and Leonard Arthur were born in 1914 and 1920 in Saskatchewan to Benjamin Laybourne (my grandfather) and his wife Ethel.

Option 2: The Korah/Algoma Ontario "Rhetta".

Ancestry users appear to default to a "Retta" McMartin (1894) who in 1911 Census was living with her widow mother Jane McMartin. Jane listed at line 32 as being the sister-in-law and and Retta at line 33 as being the niece of the head of household William Allard. This Retta married John Allard (her cousin) on July 3, 1912. With Retta living until 1977 and John Allard to 1974 in Ontario.

Unlikely to be this option given the relationship between Retta and John. Note, marriage certificate of Retta and John dated July 3, 1912 below.

Option 3: Someone unknown?

So, who is she?