Just a thought…
If you know where George Spencer Wallis was buried in 1924 (South Australia) you can contact the cemetery and see if they have a record of who the grantee of his grave was at the time.
If it was his wife, Mary Wallis, this could suggest she may have been there at the time, conversely if it was his daughter Ruth, or somebody else, this could suggest Mary Wallis may not have been there.
When Ruth Elizabeth Wallis married in 1926 (South Australia) there is a record of her foster parents.
Have you followed up that avenue at all as there may be some record somewhere mentioning Ruth's parents.
From the engagement notice, although unclear, it could be construed that Mary Wallis was not at the
wedding of Ruth in 1926.