Henry's death was registered at no. 14 they had lived there at least since 1911 census.... It was Henry's son Hugh that was with him when he died which does make me wonder if Elizabeth was still alive then or not.
I have looked for death registrations, surname Jones, with the address given as Well Street, Llandysilio. In most cases the house number is not given. These are taken from a file of transcribed death registrations for Anglesey.
Among them is:
Elizabeth Jones, 35, died 8 May, 1911 at Well Street, Menai Bridge, Llandysilio. She was the wife of Henry Jones, mariner. The informant was Henry Jones, widower.
Edith Annie Jones, 4 (but should be 4 months) died 23 July, 1904 at Well Street, Menai Bridge, Llandysilio, dau. of mariner Henry Jones. Informant Henry Jones, father.
(Edith Annie's birth registration has b. 6 Mar 1904, Llandysilio, dau. of Henry Jones & Eliza Williams.)
Ellen Jones, 42, died 13 Sep 1925 at No. 2 Well Street, Menai Bridge, Llandysilio, domestic cook, spinster. Informant M E Jones, sister in law.
I think there are others which may also interest you but unfortunately I am very pressed for time these days.
By the way have you considered buying a copy of the book of Memorial Inscriptions for St. Tysilio churchyard produced by Gwynedd FHS? It is more expensive than many of their books, presumably because of the number of graves, but may have more details about the grave mentioned in your original post.