Manx Genealogy

Re: John Stevenson & Ellinor Kewley m c 1745

Steen, Stoan, Stean are all Manx variants on Steven/Stephen - the' son' would likely be dropped by a Manx speaking parish clerk as roughly equivalent to the Manx 'agh' suffix (equiv here to 'of family') eg Boyd in English Boddaugh in Manx
thus you have the correct marriage - there will no doubt be further name variants as you move away from 100% anglicised names