I don't know whether you have followed up Mary Cottier's mother Mary Gelling who married Charles Lawson (from England). If not, she is in the IGI under Cavendish because she was born pre. her parents' marriage later that year (1788). Her parents were John Gelling & Isabel Cavendish, and Mary was bapt 6 July 1788 Braddan.
There are some messages about this Gelling family in the archives on this board, at:
www.isle-of-man.com/cgi-bin/interests/genealogy/bulletin/index.pl?noframes;read=519035

Sue