I've never heard of 15 Bayswater Road in connection of either Barrie, the Llewelyn Davies or the du Maurier's.
Barrie and Mary Ansell first lived in South Kensington (Gloucester Road) before moving to 100 Bayswater Road. Before his marriage, Barrie lived in various rooms at different addresses (mostly in Bloomsbury), but I've never come across 15 Bayswater Road.
George du Maurier and his family lived in Bloomsbury before moving to Hampstead. Gerald du Maurier lived in Regent's Park before moving to Hampstead.
Arthur and Sylvia Llewelyn Davies did live at 18 Craven Terrace when they first married and their first son George was born there. It was a house then, but it has probably been converted to flats now. They then moved to 31 Kensington Park Gardens and later at No 23 before moving out of London to Birkhamsted. After Arthur's death, Sylvia and the boys moved back to London, to 23 Campden Hill Square.
The initials JMB were etched onto the glass pane of a dining room window in the Harris Hotel in the Western Isles. You can see a photo on the database https://jmbarrie.co.uk/photos/barries-initials-scratched-into-the-glass-of-the-nbsp#more, and read more about it.