Very interesting, many thanks, Frances.
But it has created a new puzzle! (Don't worry, Tony, not in your direct line.)

This was the family I had under
5. James Corkill 1707-1779. The sons of "James Corkill & 2nd wife Catherine [possibly Curlett]": now know CROW not Curlett, were:
Robert s/o James Corkill/ Catherine bapt Parish Church 25 Dec 1745
Edward s/o James Corkill/ bapt Parish Church 20 Mar 1747/48, Parish Clerk d.1820 [will]
James d.1828 marr. Bahee Lawson d.1815 [will]
John marr. Ann Kissag 1765, d.1821.

These four brothers were all mentioned in the deed you summarised and "court noted that Edward being dumb ...".

But this was Edward, Parish Clerk and schoolmaster! Could there have been two Edwards, and CR chose the wrong one to be the parish clerk? She wrote Edward Corkhill (his spelling) 1747-1820, appointed Parish Clerk 1776."He was a younger son of James Corkill Ballasloe and his second wife Catherine Curlett. He himself was also married twice, the second time to Margaret Christian, Baldromma, but had no children." (I don't know where I got the two Margt Christians from.)
"For a time he was also schoolmaster in Maughold Village or Churchtown, supplementing the efforts of old John Brideson at the Jalloo. His death from smallpox ...." etc.

Your summary was very similar to the Radcliffes' one, except you have given the refs, and they didn't mention the deed saying that Edward was dumb (and deaf? - think that was usually the cause at that time).

His 1820 will might throw some light? CORKILL, Edward 1820 Maughold A 80 0106242

Burial: X Edward CORKILL Feb 18 1820

Burial: X Mr Edward CORKHILL Clerk of This Parish 44 years May 21 1820.

There seems to be only one will.

Sue