Yes it is the same family. William left his wife and adult children in Scotland when he came to Canada with his son Joseph. I have letters from the children who remained there to their father here in Canada. Specifically a letter telling him of Janet's death in 1850 with a lock of her hair inside the letter. She is buried in Carluke Cemetery Lair #453. I have many items that William saved in a black box and was handed down the generations to me.
His relationship with Agnes and the birth of John in Carluke is a guess. As William Fell is listed as the father on John's death record. Our family also had copy of the Speech John gave at the 7th Annual Soiree of the Natives of Carluke held in Glasgow in 1871 so I know there is a connection there. Joseph named one of his sons James Haddow Fell, my G Grandfather.
William's wife Janet was born 03 May 1779 in Easterseat Carluke (Parents John Haddow and Betty Robertson). She was actually 71 when she died. The age of 49 listed on the OPR is incorrect.