On second thoughts ....
Thomas Stevenson and Elizabeth Sloan are in the 1861 census at Ardmalush, North Bute. Thomas' age is given as 46, which suggests he was born in 1814/1815, and he was born in North Bute, as were his children Thomas, 12; Elizabeth, 11 and James, 8. His wife was born in Renfrewshire.
There is a death of Elizabeth Stevenson, daughter of Thomas Stevenson and Janet Lochhead, in North Bute in 1879, and a James with the same parents in the same district in 1876.
So there seem to be three couples
John Stevenson and Margaret Lochhead, married in Carmunnock in 1797
Thomas Stevenson and Janet Lochhead, married in Neilston in 1798
Thomas Stevenson and Margaret Hopkirk.
Therefore the one in Bute is a red herring, for which my apologies.
Errors do occur on death certificates but could Thomas' son really have got his own mother's maiden surname and the given names of both his parents wrong?