Your maternal side is a lot more tricky all round isn't it.
From what I have read of everyone's research, some of it added as links to this thread, you have the names Galbraith and Short.
In essence, question mark on the birth of Robert Galbraith born in England c. 1839 (not Scotland, according to 1841 and 1851 censuses for him and adopted family/Uncle Richard Short and wife). He was given the surname of Short by his Uncle Short. This line then became Short, although Galbraith was used as a middle name for children down the line. He may have been illegitimate and the issue of a Sea Captain father just a story. It is thought that Aunt Short might have been Galbraith and there is a Scottish link to her. He left for St Helena to live and trade there (likely died there too?).
He married an Elizabeth Denton, school mistress from Hull England, on 23rd June 1863 in Jamestown, St Helena and had children. Not clear when Elizabeth died. I found potentially an additional daughter, Jane born c. 1890, not listed in that earlier list of children for Robert and Elizabeth. If correct, maybe Elizabeth died after this year.
One of their children, named after his father, a Robert Galbraith Short born on 13 July 1875 in St Helena. This we think is your grandfather (although you thought he was born closer to 1869 - the dates quoted look specific and likely to have been researched from St Helena registers - you would need to double check these sources really to satisfy yourself on dates etc).
We have your Grandfather showing in England as a student in 1891 - linked above from the English census/family search.
At this point, I cannot see any more trace of your grandfather, nor his son, your father Leslie William Short's birth or marriage.
When I asked if you knew the name of your grandfather Robert (b.1875) Galbraith Short's wife, you are getting confused with your gg grandfather Robert (born c.1839). We know it was Robert (1839) who married Elizabeth Denton and is your gg grandmother on your mother's side. She was mother to Robert b. 1875. So we have:
Robert Galbraith Short b. 1839 m. Elizabeth Denton in 1863 St Helena
Robert Galbraith Short b. 1875 in St Helena m. who/when/where?
Leslie William Short b. circa 1911 but where? and married Loiusa Bothney Von Kerken but when and where did they marry?
That is as far as I can see!
Monica