It's very common indeed for there to be no record of a death before the start of civil registration in 1855.
As I understand it, the Kirk was interested in baptisms to ensure that every child was received into the kirk, and in marriages for the prevention of sin, but it wasn't necessary to mark a death as such because that was simply a transformation into a better life. So you only get death records if they were generated as something else - payment for a mortlcoth, a gravediggers' notebook, a cemetery record, a gravestone, a poor law or hospital record, a will, a sasine, a newspaper announcement and so on.
Christian Munro almost certainly died before 1851, as she is not in the 1851 census with her son and daughters. Nor have I found her anywhere else.
I don't think you are ever going to be certain about the parentage of John Ross or Christian Munro. You don't know where or even roughly when John Ross was born, and for Christian you have an approximate* date of birth, and you know that she was not born in Morayshire.
*As I said somewhere upthread, adults' ages in the 1841 census were supposed to be rounded down to the nearest 5 years, and the census was taken on 7 June 1841. So if 65 is accurate, Christian would have been born between 8 June 1771 and 7 June 1776. There are 9 baptisms of Christ* M*nr* between 1771 and 1776 on Scotland's People, of whom 3 were baptised in Ross and Cromarty. There will be many more whose baptism records, if they ever existed, have not survived.