While Margaret Robson might have said she had been born in Keith, all her elder siblings up to and including Helen were born in Mortlach, in Broomend or Burnend - see FreeREG.
And FreeREG shows John Robson bapt 27-3-1746 Mortlach to James Robson & Jean Gordon born at ... Burnend.
If you can also establish that James Robson was a shoemaker, that's pretty well cracked it. Have you checked out the various Tax Records at ScotlandsPlaces - those for the 1790s?
For the record, if the above doesn't hack it, there is another candidate:
- John Robson born 20-1-1749 Achoynany [sic] Keith to James Robson & Janet Bonnyman
There's one Janet Bonnyman candidate (just):
BONNYMAN JANET to JOHN BONNYMAN/ELISABETH STRACHAN
Janet BONIMAN Baptism 26 Sep 1711 Deskford
Janet BONNYMAN Baptism 6 Oct 1711 Cullen
(two records, one person)
Regarding, Margaret Sivewright, FreeREG shows there are two so named in Mortlach at the same time producing children. So watch it! Your Margaret Sivewright would have been born abt 1750. There is one in FreeREG - 1749 in Marnoch (unconvincing because the father's named Peter).
FreeREG often seems to be overlooked as a resource (and that includes me, by the way) but when there is coverage, it can be invaluable.
Hope that provides you with a suitable sized sledgehammer to batter your brick wall!