Hi David,
I was hoping the 'all-knowing' Lodger would see your post and assist given that your query is Cambusnethan based. In your list of baptisms, son Matthew Gardner wasn't mentioned but perhaps you know of his existence? He died in June 1874 aged 90, a resident of Cardross, occupation Factor and son of Matthew Gardner and Margaret Martin. He is buried in the Old Grave Yard at Cambusnethan which leads me to believe that the Gardners had a substantial family plot given the distance travelled between Cardross and Cambusnethan for burial. Would his body have been transported by train perhaps? I don't know what newspapers were available at the time but likely this death and transportation of the body may have been newsworthy?
Matthew Jnr's brother, Alexander was buried in the Old Grave Yard 19 May 1875, likewise their sisters Agnes Smellie buried 02 Sep 1880 aged 84 and Isabella Reid buried 16 Dec 1821 aged 44.
You would think it highly likely that their parents are buried there too but its possible there are no records remaining or that a headstone (if they had one) was broken, damaged or illegible. There are many family trees posted on Ancestry but the information you seek is not available there either so you may need to dig a little deeper. The Motherwell Heritage Centre may be able to help you.
Good luck with your search,
Sage.