Good heavens, Robert Nimmo and Bethia Marshall certainly obeyed the biblical injunction! 15 children over 28 years, poor woman.
Their first son was Henry and their second daughter was Elizabeth; their first daughter was Mary and their second son was William. Their third son was Robert.
So if they followed the naming tradition you'd expect Robert's parents to be Henry and Elizabeth, and Bethia's to be William and Mary.
In which case Robert's baptism is missing from the surviving records because the earliest baptism of a Robert Nimmo with father Henry is in 1777 (probably Robert's grandson).
They did name a son John, but he was only the sixth son.
There is certainly a very short window during which Bethia could have been born. Her 15th and youngest child was born in 1782, so she can't have been born before 1732, and probably a year or two later; and although she could legally have married when she was 12, it's very rare for girls to marry at such a young age, and realistically she was probably at least 16, which would mean that she wasn't born after 1737.
If she was born in 1734, she had her last child at 48. Not absolutely impossible, but very unusual.
Incidentally a Henry Nimmo (1827-1891) married a distant cousin of mine. His parents were Henry Nimmo and Margaret Rankin.
I agree with GR2 that witnesses' names can be very useful, if they have been recorded.