You'd mentioned a 1795 child earlier but I hadn't pursued this properly. He's William.
There's also a William bp 9 Dec 1796 Maughold, s/o the other couple William Coulthard & Elizabeth Callow. So there are two of them very close together.
In 1871 at Whitehaven there is a William Coulthard 74 (ie b 1797) b IOM with wife Esther 58 and nieces Catharine Bridson 33 and Christianne Campbell 21 plus a visitor Jane Eliza Lace 15. All of them born IOM. The FMP image is poor plus the January sun is streaming into my window, telling me to go outside and enjoy it while it's there! May come back to this later.