Re the Robert Gelling of Knowsley Lancs, Farmer, born IOM (1891 says "Douglas IOM"):
He's an interesting cat because he's hard to figure out.
1851: can't find
1855 March quarter Liverpool: married Catharine Kenyon widow, nee Gore. (this marriage cert would be useful to see).
1861 age 38 (b 1823) with Catherine age 44.
1871 age 49 (b 1822) with Catherine age 54.
1881 at Gellings Road, Knowsley Lancs age 61 (b 1820) with wife Catherine age 64. [lucky guy got his road named after him!]
1891 widower age 66 (b 1825).

I currently have him tentatively as the Robert G bp 15 Sep 1816 Marown son of Thomas Gelling & Margaret Creer, but if it's right then he's lopped a few years off his age.
But then it could match a Robert "Gilling" Widr 85 Living on own means b IOM, lodging with a Bertram family (shipwright b Jersey) in Toxteth Pk in 1901.
Also a death in Dec quarter 1909 age 95 W.Derby. But this might be a different Robert.

I can also buy your theory that he's the one b 1827 son of Robert G & Margaret Crellin. That would make him about 12 years younger than his wife Catherine and younger than any of the census's. He might have added a bit to make his age closer to his wife's?
Bottom line: I don't know!

Regarding the son in law William Robinson being so old, 27 years older than Martha, it may be correct. I'd hard to search a name like "William Robinson". He died before 1881 census, as Martha is a widow.

Re Jno Gelling Mason, I don't think the connection would be to the s/o Wm G because he'd be a nephew of the deceased and also only 17. My thought is that it's the eldest son of Robert b 1818 but perhaps moved away before the 1841 census and before completing the administration. Hence, they were trying to wrestle the administration away from him with the petition. Like you, I couldn't find a mason named John Gelling in the census. That's my hypothesis, without any hard evidence.