There is a possible previous marriage for Edward F Mercer (to Jessie Bannister, 1922 in Tonbridge). I can't easily see what became of her, but if it is the same Edward and he had a living spouse it would explain why he apparently did not marry Mary Kathleen.
Update: it very likely is the same Edward Frederick Mercer born in Weston super Mare, Somerset on 7 May 1892; his school alumni records (St Lawrence College, Ramsgate) place him as living at 8 Modest Corner, Southbridge, Tunbridge Wells in the early 1920s.
Later, a passenger list shows him as a "timber expert" returning to the UK from St John's Newfoundland in August 1925 - UK address given as 7 Mount Zion, Tunbridge Wells. The corresponding Canadian immigration form for his arrival there in Dec 1924 (intended occupation: lumbering) confirms his birthplace as Somerset and his next of kin as his wife, Mrs Jessie Mercer, of 7 Mount Zion, Tunbridge Wells.