I note with interest
1841 Census: George L Thorburn, 21, painter and glazier journeyman, born Midlothian, in household with John and Elizabeth Monro at 4 Carnegie Street, St Cuthberts. Also Walter Forman, aged 12, and Andrew Forman, 7.
1841 Census: William Thorburn, 15, baker apprentice, in household of John Herdman, baker, wife Margaret Alison and children.
1841 Census: Robert Thorburn, aged 14, a pupil in George Heriot's Hospital, who could be the son of Marion Lee. Do Heriot's have pupil records? Are they available for research?
Can't find Agnes b 1824.
1851 Census: Elizabeth Munro, 51, widow and Andrew Forman, son, 16 at 1 Simpsons Court.
1851 Census: George L Thorburn, 31, painter, in household with wife, children and William T, brother, 26, baker.
Marriage in St Cuthberts 18 July 1838 of John Munro to Elizabeth Thorburn or Fornan, other name Walter Thorburn.
Baptism of Walter, son of Andrew Foreman and Elizabeth Thorburn, in St Cuthberts on 21 August 1829.
Death of Elizabeth Munro or Thorburn, 69, Canongate, 1865, mother's name Bold.
Deduction: Elizabeth Thorburn, born 1795/1800, married (1) Andrew Foreman and (2) John Munro, twice widowed, died 1865, parents Walter Thorburn and ? Bold.
Comment: The dispersal of the three boys in 1841 would fit with the deaths of their parents in 1838 and 1841.
Questions: (1) Is George L Thorburn the son of William T and the recently deceased Marion Lee? (2) Why would he be living with an Elizabeth Thorburn if they are not related? Coincidence?
Speculation: Elizabeth is the sister of the William T who married Marion Lee. If so, then her death certificate will provide the names of the parents of that William T.
PS There are 10 deaths of George Lee Thorburns 1855-2013, one of whom was born 1820 plus or minus 2 years. If he matches the George L in 1851 and is the son of William T and Marion Lee, I think that would almost clinch William's identity.
PPS William Thorburn, baker, 72 St Leonard Street, died 29 December 1879 and left a 9-page will. There is one match in deaths index for William T, born 1824 plus or minus one year, death registered in 1879. If he is the son of WT and Marion Lee, I think that would be sufficient evidence for his father's parentage.
Action: Get those three DCs from SP and see what they say.