as you have probably found there is no obvious contender either for baptism or parental marriage in the Manx IGI.

The name Callow is also found in Eastern England (Lowestoft area) eg Wm Callow artist d .1905 is not Manx.

The Island Callows are mostly from Maughold and had a number of Quaker families in late 17th C. However the name is found elsewhere - there were Callows of manx descent in London early 19th C (eg the family from which Edward Callow author of 'From King Orry to Victoria' descended (see www.manxnotebook.com/fulltext/cw1899/preface.htm) - thus it may be worth checking non-manx records