You need to go to
www.scotlandspeople.gov.uk and invest in a few credits at modest cost.
There is no marriage of a Mary Dunlop or Mary Rigby to a Mr Forbes in the index there, or in the England and Wales indexes at
www.freebmd.org.uk. So Mary's second husband must have had some other surname. Nor is there a marriage or a death of a Viola Clyde with any surname at all in the Scottish index. I can't do a similar search in the England and Wales indexes because they abbreviate middle names to an initial.
You need to use one of your SP credits to find, and 5 to download, the marriage certificate of David Peter Rigby to Mary Dunlop. They were married in 1906. This will tell you how old Mary was, and the names of her parents. Use this, and her place of birth from the 1911 census, to find her birth certificate.
Then use this information to look for Mary's death certificate (assuming that she stayed in Scotland), which should be indexed under her maiden surname. This will tell you the name of her second husband, and you can try for Viola under her stepfather's surname elsewhere.
If they had been divorced in Scotland, I would have expected to find a listing of the divorce papers in the online catalogue at
www.nas.gov.uk, but there is no such listing. Therefore it may be that they were divorced in England or somewhere else, or that Mary died before 1918 (though if she had several more children that seems unlikely). Or Mary may have set up home with a Mr Forbes but not actually married him.
