the Manxbmd provides a partial index into civil registrations which exist from late 1870's - the full records would be held at general registry who charge for access - the Manx museum has copies of indices for the early part of Civil reg period (I think approx to 1920 ) - parish registers and many Methodist records for this period have been transcribed or held at Manx Museum.

Marriages could have been off Island

Manxbmd has a birth of Ann Jane Christian Wilson for the year 1904 (search births) but you need buy a birth cert to get full details.

Are you sure you really have checked the newspapers ? I would have expected at least a notice of the marriage + possibly that of the death of John Duff.

If you are not on Island I recommend you arrange for a short break in Douglas where you will find it much easier to talk to staff at Museum - joining IoMFamHist Soc should also provide some help from their library volunteers