If we look at the information a little closer it unfortunately poses more question than supplying answers
I am at my last ditch effort to find information on Charlotte Caldwell. Scotlands People has no birth record, no death record for her.
The main Old Parish records on Scotlands people are for the Established Church of Scotland with Roman Catholic records added fairly recently- in the main, birth records are reasonably well kept by the churches although there are exceptions and many records have disappeared over the years.
It is also possible that Charlotte was of a different church although the main schism of the established Church did not occur until the 1840's there were a number of other breakaway groups, each of which formed their own churches and maintained their own records.
Walter Cathcart married Charlott Caldwell on July 15th 1800 in Abbey Renfrewshire.
Does this record show it as an actual marriage or is it a calling of the Banns (normally called the 3 Sundays preceeding the wedding in both the Grooms and Brides Home Parishes.)
If the notice you have is for the actual wedding then it is probable that Charlotte came from Abbey Parish as it was more normal for the wedding to occur in the Brides home parish.
With regard to Silk Street and Smithhills - both of these areas were levelled in the mid to late Victorian era and tenement buildings were erected which encroached right up to the Abbey burial ground. The Town Hall was constructed later opposite the Abbey and many properties were levelled to allow this, looking at old maps it is also possible that part of the burial grounds were also covered over. The Victorian/Edwardian tenements were in their turn demolished over the years with the last of them going in the 1960's to leave the Abbey in open space for the first time in a number of years.