You wondered what happened to Susannah Webb nee Bridson. Did you find her in the IOM 1911 census? I couldn’t find a death/burial either, but I’ve only looked on Lawson’s Microsite for the IOM and Ancestry for possibles in London.

From Ancestry:
(1911) 7 Strathallan Crescent, Douglas
Susan H Montgomery, wife, 28, married 5 yrs 1 child, living, Boarding house keeper, born Douglas IOM
John Montgomery, son, 4, born Douglas IOM
Susan Webb, mother, 50, widow, born Castletown IOM

From Lawson’s microsite:
WEBB, Susan Hannah married 1905 D PRES 27 MONTGOMERY, Robert

WEBB, Susan Hannah birth registered 14 Aug 1881 Douglas 325 d/o WEBB, Robert & BRIDSON, Susannah

There was this death in Liverpool of about the right age, but you’d have to know of a connection with that area to make it worth looking into.
Susannah J Webb born abt 1857 died Mar 1935 Liverpool South Lancashire

There were numerous Susan Webbs around the right age all over the place – you’d need to see for yourself to see if any likely locations jump out. But I expect you’ve already tried that.