I looked at these Corkill marriages last night, but they didn't make things much clearer.

These were possible Jane ......? w/o Robert Corkill named in the deed:
Robert Corkil / Jane Collister 25 Jan 1772
Robert Corkil / Jane Scott 15 Feb 1772
Robert Corkil / Jane Callow 23 Nov 1776, Robert a widower.
Jane CORKILL als SCOTT als ALLEN buried Apr 18 1786.

These were the only children I could find at the right time with mother Jane named (lots with father only):
CORKIL Ellinor d/o Robert Corkil/ Jane Callow Parish Church 24 Jun 1777, Private baptism
CORKIL James s/o Robert Corkil/ Jane Callow Parish Church 28 Feb 1779 (possible burial which follows James C Ballasloe May 8: James CORKILL infant Aug 2 1779).

A Robt Corkil buried Jul 25 1780.
CORKILL, Robert 1781-2 Maughold E w ? Or too late?

CR thought the parish clerk/ex-schoolmaster married Margaret Christian (presumably from his will), but we don't know who the Ballasloe Edward married, if anyone (and assuming he wasn't the schoolmaster/p.clerk as CR thought).
Marriages:
Edward Corkil / Margaret Christian 16 Oct 1777
Edward Corkhill / Margaret Christian 7 Jun 1815. "Mr Edward Corkhill widower & Miss Margaret Christian". [Definitely the parish clerk, and this was his preferred spelling of his name:
For example: William Kerruish / Sarah Corteen marr.27 Jun 1818
Witnesses Edward Corkhill, William Kerruish & Robert Corteen.]

So instead of two marriages of Edward parish clerk with both to Margt Christians, perhaps the other Edward? Except that the par clerk was a widower.

These were the only bapts I could find at the right time:
CORKILL John s/o Edward Corkill/ Jane Creer Parish Church 30 Jan 1785
CORKILL Ann d/o Edward Corkill/ Jane Creer Parish Church 8 Jun 1788
CORKILL Mary d/o Edward Corkill/ Jane Creer Parish Church 30 Nov 1790, Private baptism

So this could be "dumb" Edward?

Any chance of looking at the 1820 Arch will?

Sue