Wilros: You're getting confused by terminology.
A sheriff- substitute at that time was a legally qualified judge, of lower rank than a sheriff, but usually a post given to an advocate with substantial landholdings in the area, after he had spent some yerars practising law. The term "sheriff-substitute" was a misnomer, as they were the judge for the locality, subject to the direction of a sheriff who sat at a head court many miles away. The terms were replaced by the more accurate "sheriff" and "Sheriff principal" by the Sheriff Courts (Scotland) Act 1971.
A sheriff officer is the man who cites witnesses, serves writs and enforces court orders for debt, similar to an English bailiff. It is not a job requiring a legal qualification beyond a basic exam on the practice of the trade. It's a job commonly done nowadays by ex-cops, but in the 19th century might be done by a former soldier.
Therefore sheriff-substitute James Campbell and sheriff officer James Campbell are not the same man.
The James Campbell servant in Borley in 1822 could conceivably be a sheriff officer in 1841, although "Kitty" is more likely to be "Katherine", and he would need to have moved up in the world to jump from being a servant to a server of court documents. It's far from likely and the name is a common one.
The 1822 marriage appears to be a common law Scots irregular marriage being formalised by warrant of a court. This type of order has been discussed on Rootschat before.
Moving on to Christian Dingwall, I agree that the most plausible explanation is that her baptism record is missing. There is a rather surprising gap in the records for the second half of 1798, with eighteen baptisms from 5th January to 18th July, and only three for the rest of the year. This might imply that a page of paper records has been missed from Hew Morrison, and perhaps been lost completely. While a drop in births could be the result of catastrophe, I'm not wawre of any record that would explain it and the date fits your hypothesis.
Finally, migration to the west and south was becoming common by the early 19th century, so a departure to Canada or elsewhere is certainly feasible. Records by name are not readily accessible online.