Early Life
Mary was in her "90
th year" when she died in 1842, meaning she must have been born after Sep 1751. There is one baptismal record that is potentially the right person. This is Mary Bond born 21
st September 1751 at Caldbeck in Cumberland, to a Quaker family (Joseph Bond and Ann ?). Caldbeck is quite a long way from Carlisle and so it's not immediately obvious how she would have met Isaac, nor why they would have married in St Cuthbert in Carlisle rather than in her local church (unless perhaps she was turning her back on Quakerism and hence didn't marry there).
One contradictory piece of evidence is in her
marriage record, which gives her age as 26 (in 1780) and therefore a year of birth of 1754. Maybe what we have here is some intentional, or accidental, misleading of the officials (and maybe her future husband) as to her age. I still stand by the obituary implied year of birth of 1751/52.
Death
Her obit mentions that she was the mother of "Mrs Bell, of Picton Place" (i.e. her daughter Catherine). The place of death is given as "Newbiggen, near Carlisle". This was presumably a farmhouse because the 1841 census shows her there, living/staying with her eldest son Thomas, whose occupation is listed as "farmer".
From the Newcastle Journal (1 October 1842):
At Newbiggen, near Carlisle, on the 13th ult. in her 90th year, Mary, relict of Mr Isaac Lowthian, formerly of that place, and mother of Mrs Bell, of Picton-place, in this town, much respected