Hallo Averil,
I think the the John Corkill bMarch 1773 who you have found lived, married, had family in Liverpool from where he returned to the IOM in his later years. Its unfortunate from research point of view that he was born same year as the JC who also left the island but in his case to came to Penzance - I think the two of them quite often get mixed up.
You are right about the different names - over the years I have come across many including Cockle and various spellings of Corkill
Can I just check where you found the Kalkhill version for John b1773 ?
Whats your interest in the Penzance family - are you related?
Alison