For missing 1871 census, look for "William Culbon". He's at Hulme, age 30 Tailor born IOM, with wife Annie E 31 born Hooley Hill Ashton U. Lyne and 3mo son William R born Hulme.

I can follow your logic and it makes a lot of sense. William Cubbon born ca 1811 fits well to be the William bapt 22 Sep 1811 Braddan to Thomas Cubbon & Mary Kelly.

IGI marriage Thos CUBBON & Mary KELLEY 17 Jan 1807 Braddan. No marriage in range to a Margaret Kelly.

There seem to be 4 children born to this couple in Braddan, all Mary Kelly:
1. Thomas 7 Feb 1808
2. Margaret 17 Sep 1809 (she fits to the one aged 30 in 1841)
3. William 22 Sep 1811
4. Jane 31 Jul 1814

Then they move to Santon:
5. Ann 11 Aug 1816 (to Mary Kelly)
6. Cath 6 Mar 1820 (to Margt Kelly)
7. Eliz 7 Jul 1821 (to Margt Kelly) she matches the one aged 20 in 1841.
8. Isabella Cabbon 27 Dec 1823 (to Margt Kelly) she's 18 in 1841.
9. John 27 Aug 1826 (to Margt Kelly) he's 14 in 1841.

A glitch is a Jane Cubbon bapt 27 Feb 1814 in Santon to Thos Cubbon & Margt Kelly. No idea who this is.

I think Mary Kelly and Margaret Kelly are the same person. Perhaps the Santon clerk had a flourish at the end that looked like "gt" instead of "y".

Supporting this is that there's a Santon burial for a Mary Cubbon 29 Nov 1829. No age and no will unfortunately.

Whether the father Thomas born ca 1777-78 is the right one to match the son of Wm C & Cath Gelling remains speculative, as there are other Thomas Cubbons baptized around the same time.

The Isabella Gale or Gill als Fell widow of James Gill, who is buried with Wm & Ann Clucas age 87 in 1841, and who leads the 1841 census, could be very interesting. At the moment, I'm stumped because I can't find a marriage or any children on the IGI using different combos of last names. I'd guess she's connected to the Corkills, but not sure.

I'll try some more in the next few days.