Cait McPhee has now confirmed the details outlined above. She wrote:
"J64, according to his death certificate (1866 in Govan) was the son of Peter Mutch (P1) and [unnamed] Chapman as you say on your website, and the death certificate says he was married to Margaret Simpson, and was a "shipbuilder's gatekeeper". However, there's then a death certificate for Christian Simpson in 1870 in Tradeston in Glasgow, widow of John Mutch, shipbuilder's gatekeeper.
So it looks like John J64 marries Christian/Margaret Simpson in 1824, they move to Glasgow, and Christian outlives this John. Their son, John (b. 1824) witnesses her death certificate."