I checked my notes, and recalled that I found a William Cowin to match the one at 4 Great Nelson Street: born 1799, parents Edward Cowin (probably the one born 1766) and Jane Taggart. This couple had a son John in 1802, who was the same age as the one that married Clague.
Wills mentioning this Edward are on the Manx Notebook site, but the coverage stops too early. It would have been more useful if they had covered wills from about 1741 to 1841, instead of 1700-1800, then we would not be left with 40-50 year gaps to fill before the 1841 Census.
Cheers,
Stephen