I hope you are sitting down, because the plot is thickening with some startling results
I downloaded Samuel Loag and Mary Ann Muir Murray's marriage certificate. His occupation is a coachman, which fits with the 1891 and 1901 occupation of van driver. He must have become a photographer's manager later on.
His parents are Samuel Loag Iron(molder?), deceased and Mary Loag M.S. Martin!!!! Help!
That was the name of the mother of wee Nellie... and perhaps she had married the William Smith, brassfinisher after her husband Samuel died in 1880.
So it looks like Nellie/Helen Loag was actually Samuel's half sister, rather than his daughter!
He and Mary Ann must have taken her on and raised her as their own daughter. Trouble is, the real father is never mentioned by name... On the birth cert, which turns out to be Nellie after all, she just states that her husband is not the father of the child. Helen must have grown up thinking that her mother was really her grandmother and her brother was really her father!