Looking at the information so far, would certainly look likely that birth entry for Sarah, daughter of John and Agnes, was the wife of David McKenzie

I can't see an Old Parish Register for their banns/marriage unfortunately but likely this was before 1851 given that likely 1851 entry.
Re deaths, struggling to see them as yet. Not sure which line you are following (John or Alexander) but would be useful to check both their marriage certs to see what they say regarding parents and whether they showed as alive or deceased at the time of their marriages.
Monica