Back to George Simpson and looking at his military record.
He attested in March 1900 in Edinburgh. As far as I can see, he did not go abroad until 9 Nov 1901 when he was sent to South Africa until 1904, and then to India, before returning home in Jan 1908. (He also served in WW1). In the 1901 census, he is in military barracks in Newcastle on Tyne.
I suppose he could potentially be the father of Agnes, if he had home leave during August of that year.
However, it seems more likely that Alexander Simpson born 1884 might be the father.