I don't know if this helps, but I find I have a baptism for an Isabella Cavendish (well, Corjeage) of the right age to marry John Gelling in Braddan in 1788 after all. I don't know where I found it because in the space where I would normally write IGI, Manx notebook or Register I've got 'SS' and I'm afraid I haven't a clue now what that was supposed to represent.

Isabella, d. of William Corjeage & Isabella Moore c.29 Dec 1766 St Peter Liverpool (SS)

There is a possible marriage for her parents:

William Curjeag m. Isabel Moore 7 Dec 1759 Braddan IGI ext

and a couple of other children:

Wm Curjeag s. of (?) John Curjeag & Isab Moore 7 Sep 1760 Braddan IGI ext
Thomas Corjeag s. of William Corjeag & Isabel Moore c.28.11.1762 St Matthew IGI

None of these has been checked with the registers yet, I find.