Its all a mystery to me,
According to the evidence you have found so far:
William Summers / William James Summers / or James Summers was born about 1861/1862.
1889 Marriage certificate living in Newport, occupation Seaman which gives his age as 28 so born about 1861.
1891 Census living in Newport which gives his age as 29 so born about 1862.
1901 Census living in Newport which gives his age as 39 so born about 1862.
1911 His wife Edith and family are living in Blyth Northumberland by the sea. She is married so he is still alive at this time. As a Seaman he is possibly on board ship.
1907 He is named on the baptism of his daughter baptised at Blyth, Northumberland so still alive.
1919 He is named as the father at his son’s wedding in Glasgow. Edith Agnes is named as mother but she is (deceased). It does not say (deceased) after his name so it would be fair to assume he is still alive in 1919.
It is possible that the family moved from Northumberland to Scotland possibly to Ardrossan or Glasgow after 1911. If they did then it may be worth looking for his death in Scotland. I don’t know much about Scottish records but I believe that the parents are named on the death certificate. It is possible that his parents names are unknown by the informant of course but worth a try. If you do look for his death then keep in mind the variations of his name that have been used in the other documents.
The Scotlands People website have all the records.