Hmmmm. You do seem to have quite a conundrum.
You have Helen Davie, born in Stirlingshire around 1817/1820, living in Kirkintilloch in 1841 when she got married, saying in 1851 that she was born in St Ninians but afterwards that she was born in Balfron or Fintry, dying in 1886, death registered by her son who almost certainly got her age wrong and who may have got her parents wrong as well.
Comparing the 1841 and 1851 censuses, it looks as if Helen Davie is the otherwise unnamed Mrs Edmond at Lernock House in Balfron with Peter E, also 25, and Thomas E, 20, who is probably her brother-in-law. However this gives her age as 25 (which, because adults' ages in 1841 were supposed to be rounded down to the nearest multiple of 5 years, means anything from 25 to 29, and if accurate implies a birth date between 8 June 1811 and 7 June 1816) and her birthplace as outside Stirlingshire. This is inconsistent with the information she gave in later years.
I can't honestly think of any easy way to progress this. If one had unlimited time and funds, one might look at all available death certificates of people surnamed Davie, in the hope of finding possible siblings (either people with parents James Davie and Agnes/Ann/Nancy or people with parents John D and Mary Wright) but even that won't work if Helen was born illegitimate or had no siblings.