Just a Thought - You know that Mary ALLCHURCH - nee BROCKWAY - passed away in 1922.
Have you obtained the Registration of Death -Long Form Death Certificate
There should be indication of Marital Status at the time - married/ divorced etc name of spouse etc.
REGISTRATION of Marriage for William ALLCHURCH & Isabella GEORGE nee HUNTER - will show Marital status of both bride and groom - at the time of this marriage.
Perhaps the marriage between William ALLCHURCH - and Mary J. BROCKWAY - had been annulled ? -