Sorry I am unable to help you with your Mary McKelvie - it has taken me a couple of years just to sort out my two Cumming lines and so I have not been able to delve into the McKelvie line as yet, but McKelvie's have married into my known Cumming lines three times.
There have also been other Cumming/McKelvie marriages from a Cumming line whose ancestor I have been unable to find.
I find your reference to Christina Ferguson interesting as Christian has been used in both of my lines several times, and my GX3.Grandfather, William Cumming, married a Mary Ferguson (she died aged 32 in 1792), prior to his marriage to Janet McKelvie. William and Mary Ferguson named their daughter Christian.
It seems possible that we may have a link, but it will take some researching to find this.