The California Death Index for Elizabeth Scott Colquhoun who died 1944, states
Father: WALLACE
Mother: ROBERTSON
And of the two Wallace-Robertson couples you mention, the one with the daughter Elizabeth
Scott WALLACE is likely to be the right one?
Those parents appear also to have had:
James WALLACE b 1857, Bothwell
Thomas WALLACE b 1861, Bothwell
Mary Wilson WALLACE, b 1863, Tarbolton, Ayr
George WALLACE b 1866, Old Monkland
John WALLACE b 1868, Old Monkland
The family in 1871: at: Clarks Land Parish Road Police Station, Calderbank, Old Monkland
James WALLACE 42, ---------? b Fenwick ,Ayr
Wife: Mary 38, b Bothwell
Son: James 13, b Bothwell
Dau: Elizabeth 11, b Bothwell
Son: Thomas 9, b Bothwell
Dau: Mary 7, b Annbank, Lanarkshire (sic)
Son: George 5, b Calderbank, Lanarkshire
Son: John 2, b Calderbank, Lanarkshire
Dau: Janet, 5 mth, b Calderbank, Lanarkshire
Servant: Barbra LAMBIE 17, Domestic Servant
In 1881, the same family is at No 1 Wallace Land Calder Bank, Old Monkland, Lanarkshire
James WALLACE 54, Provision Merchant, b Fenwick, Ayr
Wife: Mary 49, b Shotts, Lanarkshire
Son: James 23, Commercial Clerk, b Shotts
Dau: Elizabeth 21, Normal Student, b Shotts
Son: Thomas 19, Shopman, b Shotts
Dau: Mary 17, Scholar, b Annebank, Ayrshire
Son: George 18, Scholar, b Old Monkland
Son: John 12, Scholar, b Old Monkland
Dau: Janet 10, Scholar, b Old Monkland
Your James must have made his way back to Scotland again after 1883.
He and Elizabeth married in 1887:
https://www.scotlandspeople.gov.uk/Scotlands People:James COLQUHOUN
Lizzie Scott WALLACE
Registered in Blythswood, Lanarkshire
Cheers
AMBLY