you had to be 21 to marry without parents/guardian's consent - judging from the MI she was 21 rather than 18 thus I tend to choice of the 1st two.
try the igi for isable christian's baptised c. 1779 - three obvious are
ISABEL CHRISTIAN 20 JUN 1779 Malew,
ISABEL CHRISTIAN 25 APR 1779 Kirk Rushen,
ISABEL CHRISTIAN - 10 DEC 1780 Malew
there are a couple of others slightly out of the time frame - welcome to the Manx tradition of a small pool of names !