Both Mary Murray's marrying within two weeks of each other, one to Peter Dawson and the other to Peter Davidson, in Kinfauns, no wonder I got lost.
It's not two different Mary Murrays marrying two different Peter Davidsons. It's two records of the same marriage.
What is in the parish registers is not necessarily a record of the wedding ceremony. It is a record showing that the banns were called in both the bride's parish and the groom's parish. Sometimes the record will mention the date and place of the actual wedding, but not always. The difference in the date is because the banns might have been called on different days in the different parishes.
The third one is odd, but the fact that it's the same date as the record in one parish and the same parish as the other date suggests to me that it's a third record of the same marriage. Have you looked at all three originals? Are the two Kinfauns one is the same handwriting? Could it be that Peter had had his banns called in Kinnoull, taken his certificate over to Kinfauns where the clerk wrote it down with the alternative surname, and then the banns were called in Kinfauns and someone else - the minister perhaps - wrote it down again?
I think the family at Inch Farm, Clackmannan is probably a red herring. There is a marriage of a Mary Edgar rto a James Dawson in Alloa in 1839, and a Mary Dawson, other surname, Edgar, died in Edinburgh in 1871 aged 57. There are no baptisms of any children, but a Joan Christina Dawson married in Glasgow in 1881 and died there, in 1884. If any one of those certificates says that Mary's husband/Joan's father was James, I think you could safely disregard them.
Peter is in the 1861 census in Forteviot with wife Mary, born Forteviot, his children Mary, Elizabeth and William, stepdaughter Ann Gowans, 9, and another daughter Janet Davidson, 8. (This is from a transcription - best to check the original image on SP)
I am stumped by Ann Gowans, because Janet Davidson is the daughter of Peter Davidson and Mary Murray, who were married to one another both before and after Ann Gowans was born. Could she be an illegitimate grandchild? Or a foster child?
In 1871 Ann Gowans, 19, born Forteviot, is in the household of her unmarried uncle James Gowans, born Redgorton, and her unmarried aunt Mary Gowans, born Forteviot. They are the son and daughter of John Gowans and Ann Morris, so there's no immediately obvious connection to the Davidson/Murray family.
Peter, Mary and Ann Gowans are still in Forteviot in the 1881 census (LDS transcription - again, check the original).
I have failed so far to find a death of a Mary Davidson, other surname Murray, born 1815 plus or minus 10 years.