Yes, I speculated that the errant father might be George Waddell of Gain. He certainly fits the bill.
He was born in Bathgate in 1801, and married Elizabeth Shanks in New Monkland in 1826. There had been Shanks in Gain for some time before George arrived. George and Elizabeth were definitely in New Monkland at the relevant time, because two of their children were baptised there in 1830 and 1832. Elizabeth Shanks died before 1841, and George married Margaret Gillespie in 1852. He died at Gain on 31 August 1868. He was certainly the only George Waddell who was farming in New Monkland at the time.
However there were others not far away. For instance there was George Waddell of Binniehill in Slamannan, who was born in 1803 and married Marion Morrison in 1834. Unlike GW of Gain, he was unmarried at the relevant time.
There was also George Waddell, born 1813 at Rochcraig, son of Robert W of Stanrigg. He would have been only just about old enough to be the father of Rebecca Stevenson's son. (Stanrigg was probably the biggest Waddell farm in New Monkland at the time.)
Also in Slamannan was George Waddell of Ballochnie and Balquhatston. He might be thought a bit old, because he was born in 1787/8, but he didn't marry until 1844. He probably isn't the one, however, because although he did own and farm Balquhatston, his primary occupation was Writer to the Signet.
There were also George Waddells in Shotts and in Old Monkland, though none of them is an obvious candidate, and any number of others from a little further away.
BTW we are almost certain that the large and prosperous Waddell family of Gain was a branch of the Waddells of Leadloch and Holehouseburn, who can be traced back to Adam Waddell, who bought Holehouseburn in 1609. We just can't quite make the connection!
