Ross and Cromarty is the name of the county, and it is informally divided into Easter Ross and Wester Ross. All the places we have mentioned in R&C so far are in Easter Ross.
I wouldn't dismiss the other children born to John Ross and Christian Munro in Easter Ross. Christian was listed as aged 65 in 1841, so she could have been anything between 65 and 69 if that is accurate, so born between 1771 and 1776. A marriage date of 1805 would fit, and the last two children were baptised in Nigg, which is where Donald says that he and Christina were born. There's a long gap between 1805 and Donald's birth year, so it would all fit together.
I have to confess to cheating slightly with the 1881 census, because I have a set of the LDS CD-ROM transcription of the 1881 census, which makes searching a lot easier once you are aware of its quirks. Ladysbridge Hospital is in the County of Banff, Parish of Boyndie. Census 1881, parish No 143, Enumeration book 3, page 10.
It's quite likely that there is no surviving record of Christian's death. The best you could hope for would be a record in the Dyke Kirk Session minutes of payment for the mortcloth. Every parish had a mortcloth (or sometimes more than one) which was draped over the coffin during the funeral service, and there was a fee for the use of it, the money going towards the funds for the relief of the poor of the parish. These amounts are often listed in the parish accounts. The Kirk Session is the committee made up of minister and elders that runs the business of the kirk. The records are mostly held in the National Records of Scotland, and the do include the Dyke KS for the relevant years. They KS minutes have all been digitised, and Scotland's People intends to make them available but so far has not managed to do so for technical reasons. In the meantime they can be viewed on-screen in Edinburgh and in a few local archives, but of course they are all closed at the moment because of coronavirus.
Out of curiosity, who were the parents of the Donald Munro who died in Dyke in 1857, and who was the informant?