Hi Denise
I have figured out your grandmother's birth but not as yet her death.
You know her age was 29 in 1933 and her name was Helen Bruce, formerly Forbes. This implies that she was known at that time as Helen Bruce but had been known previously as Forbes. No reference being made to her maiden name - suggests it's possible she had not been married but had simply changed her name for some other reason during her lifetime.
You also know there is a marriage between a Helen Forbes and a John Charles Bruce in 1904 - although this Helen is around 17/18 years older then your Helen.
BUT - what if the elder Helen is the mother of the younger Helen

Agewise it fits and may explain why the younger Helen's name had changed from Forbes to Bruce.
We know from the census in 1891 where the elder Helen's family are staying, and checking the 1901 census finds Helen a domestic servant at a farm in Kinellar and her family staying at Clintery Cottages, Newhills. Checking the marriage of Helen Forbes and John Bruce - we see that Helen's address in December 1904 is Clintery Cottages, Newhills also.
So now we need to see if there is a birth for a Helen Forbes prior to that marriage - which there is on 13th January 1904 - also at Clintery Cottages. Her birth is shown as illegitimate and there is no RCE naming her father. So either John Bruce was her father and her parents married almost a year after her birth or John Bruce was her step-father and she took his name.
The death of Helen Forbes/Bruce which you have in 1975 - what are her parents names from the death certificate - and was she married to John Charles Bruce ?
Regards
Susan.