Your find of Sarah McKilligan's death registration in Washington state with correct parents is a great find and confirms her marriage to McKilligan. There is a widow Sarah McKilligan, b. 1856 in Ontario living in Red Deer, AB with sons Roy and William in the 1916 census. The sons are listed with Father Alex and mother Mary in Alberta in the 1906 census. I suggest that this is our Sarah and that she married Alex McKilligan between 1907, the death of George Buxton and 1916.
Sarah Jane Fisher/Buxton, b.1856 is in Goderich for 1901 census and likely there until 1907 when George Buxton dies. also in the 1901 census is Sarah Buxton, b. 1864 in ON, in Vancouver. There is a Sarah Buxton, b.1863 in ON, in Winnipeg in the 1891 census. I wonder if the Sarah F Buxton, b. 1867 in ON living in Cody Wyoming for the 1910 census is not this other Sarah. The date of birth is certainly closer.
Is there any family legends or info regarding how Sarah, at the earliest age of 51 when George died in 1907, moved out west to join the Buffalo Bill Wild West Show?