There are a will and decree you could order to see whether this couple were the right parents for John (marr. Christiana), because you need proof of the descent:

CANNELL, John 1741-2 Mau Epis decree, LDS film no. 0106376
CANNELL, John 1742-1 Mau Arch no.56, LDS film no. 0106218
[From Brian Lawson's website.]

If one of these John Cannells was the correct father for your John, his will/decree might also show siblings or other identifying details to be able to work out which parish he came from, and who his parents were. (Early 1700s bapts show that two John Cannells had arrived in Maughold.)
However it is possible that the above will and decree were both for the same person.

Maughold burials have Jon CANAL Senr buried Feb 15 1741.
I couldn't find a Maughold burial for Margaret Cannell als Cannan. The one on Brian's burials index for Maughold Mar 2 1736 was Margt CANNAL als KNEAL.

Are you able to order the two wills films from a Family History Centre?

Sue