Not sure if marriage certs at this time included bride and groom's parents' names. From the Australian marriage index, Robert Jnr married Margaret on 12 Jun 1886 - ref 000426 & page no. 16316.
His death registration on 26 Jul 1931 certainly gave his parents as Robert Wright and Sarah Francis. From the Australian death index, ref 003297 & page no. 1818.
How great that you have found an obit for Robert Jnr which included his birth date. Haven't been able to see anything on the Scottish records for it so far. Not unusal though as official Scottish registration of BMDs only started from 1855.
With Kay's marriage cert for Robert Snr and Elizabeth Frew in 1856, they both certainly show as unmarried at that time. A Sarah Francis could well have been the name of Robert Jnr's mother, which his paternal granparents maybe taking care of him after his birth (as he showed with them from Lodger's entry on the 1851 census). One thing though, the grandson Robert in 1851 showed as aged 6 at the time of the census, so born around 1844-45. Not sure if you have confirmed Robert Jnr's age/date of birth in other documents too.
One thing to do with Elizabeth Frew, there are obviously issues to do with her that you would need to reconfirm maybe. As Kay mentioned, her parents' names don't match what you mentioned earlier. You mention, and I have also seen elsewhere, that she had previously also been married and had at least one child. Like Robert Jnr, Elizabeth Frew is showing as a spinster at the time of her 1856 marriage to Robert Jnr.
Monica
Added: Any thoughts anyone on the maiden name of Elizabeth Frew's mother, also Elizabeth?