Debbie, did you notice that William's age at burial and in the 1851 census means that (if correct) he married at aged 17?
The William who was the illegitimate son of Ann Dawson and Wm Benson was bapt 27 Sept 1798 in Malew, and if bapt soon after birth, which was usual, he would have been only 12 in 1810 - and seems highly unlikely to be the one who married Esther Kneale and lived in Maughold.
If it was a late baptism this was usually noted in the register, which can be seen on LDS film no. 0106730, or in the MM.
Brian Lawson's wills index has an entry which looks like Esther's will:
BENSON, Esther 1864 Mau A[rch] [No.] 95, [LDS film no.] 0106266, which should be of interest to you.
Other wills which might connect William to another family in the IOM by mentioning him and/or his wife or children are:
BENSON, Eleanor 1848 Mal A 36 0106252
BENSON, Francis 1832 Bra E 2 0106432
BENSON, Mary 1855 Mic A 54 0106257
His son Hugh and wife appear to have left wills:
BENSON, Hugh 1903 Ram 194 0106545
BENSON, Margaret 1897 Ram 5 0106525
Wills are always interesting and can be invaluable in linking people from different parishes.
M.I.s for William and Esther, and possibly other Bensons, might also help.