Hi Ken,
Was Maughold given as William Corkhill's place of birth anywhere other than in the letter from James Cashin? (Perhaps marriage or death cert?) - He calls him only "Corkhill" there, and many Manx people had emigrated by 1850.

In your message you wrote Immigration Abt. 1837, Possibly arrived Victoria on the Duke of Argyle which supposedly on the same boat as Dr Crellin’s parents.
Who was Dr Crellin?

I can't find any early records for Richard Blake Corkill in the IoM or England, and no Richard or Blake Cork(h)ill in Maughold baptisms.
He does appear in Masters & Mates certificates 1850, which gives his birth as 1813 in Whitehaven, but like you, I couldn't find him on the IGI.
The Liverpool Mercury Jan 25 1888 [British Newspapers 1710-1953] reports his death in Balmain, Sydney, on 10 Dec [1887] aged 74, formerly of Whitehaven.

I can't see a connection to William Corkill b.1801 in Maughold, Isle of Man, unless the family moved to Whitehaven following Ann's bapt in 1809, and had more children there.

Merchant Navy Seamen 1845-1854 [BT 114 piece no.5] has an entry for Corkhill Wm KirkMaughold (transcribed as Kirkmargold, but Maughold on scan, Find my Past), but no age given. If you haven't seen this you may be able to follow him up in Merchant Navy Seamen records under the No of his Register Ticket: 524712.

Sue