Hi Andrea! Yes, I sure am in Brisbane
Are you in Australia too?
I think I'm making some sense of it all slowly. For some reason I had that Alexander was born in 1833 and not 1827. This confused me when i kept seeing Thomas b1833. I can't find any information on a death of Thomas b1833 but it would almost certainly be the case seeing as your ggrandfather was named Thomas in 1840. I also have a copy of the monumental inscriptions at Blair Athol church which say that Thomas McNab died in infancy and that Alexander McNab died in Brisbane in 1844 ages 57 years - so this fits with the date of 1827. All those numbers can make your head spin!
I notice that you don't have a Jane listed in your original post - the inscription information i have lists under children a Jane Mcnab died at Chester 30 July 1881, aged 42 years. Do you have any record or her?
Did you manage to go back further than Robert's father Robert b1857?