I had seen the Margaret Kewley in Gravesend as a wetnurse when I was looking for John Kewley's sister Margaret b. 1827 @ Braddan. Haven't seen any sign of her since her appearance with the family in Santan in 1841!!
But as you suggest the Gravesend Margt seems to fit better. Can't find death / burial for baby Catharine Margaret Kewley b 1859 in UK or on the Island.
I think it is possible that John Kewley's brothers Edward and Thomas went to sea before 1861