Hi Averil,
The Margery Looney 1765 will looks like the Margaret Loony who married William Looney Maughold 6 May 1732 - interesting! (children Wm, James) - and the other William Looney who is causing such confusion was her brother, who also married a Margaret (children Peter, Jane, Isabel).

It isn't clear from the parish register whether the brother and sister-in-law were William Loony and Margaret Corkill married Maughold 15 Jun 1735, or whether there was yet another Wm and Margaret. There were a lot of children who are not mentioned in the 1765 will.
Also remember there were several Margaret Looney burials not recorded as being children. Strange that Margery/Margaret Looney d.20 March 1765 doesn't appear in my burials transcript.
Also odd that Margaret Corkill's surname wasn't given more often in the parish register if she was the sister-in-law. Perhaps the clerk was also confused by the repetitive names!
Her own will would be helpful.

Re. Daniel's 1772 decree (burial Maughold July 29 1771) naming siblings Peter, John, Willm., Jane & Isabel Looney, I think you are right, that these were children of William and Margaret (surname unknown) - luckily the names differ from the usual John & William. A Daniel was clearly Margaret Corkill's son, but this was probably his burial:
Dan LOONEY (infant) buried Mar 27 1735.

It would be good to tie up a few loose ends, such as what happened to the other Wm / Margt children - especially Joney who was the only other child clearly of Margaret Corkill's.

I think this is probably Daniel who d.1771:
Maughold bapts:
Daniel son of William Looney bapt 8 Aug 1747 Private baptism. Twin of William
William son of William Looney bapt 8 Aug 1747 Private baptism. Twin of Daniel.

More children of William and Margaret's? - the mother's name isn't given.

Sue