Well I guess we have something in common because I believe Alexander (son of Ellen Dewar and Alexander McDonald) was my GG grandfather too.
My Grandmother was Eva May McDonald born to Thomas Rankine McDonald and Eliza Arnold.
Thomas Rankine McDonald was the son of Alexander McDonald and Jane Rankine.
I have a copy of the 1841 census with Ellen Dewar and her son living with Ellen's sister Janet but by 1851 Alexander was still living with this family but there's no sign of Ellen and I don't know what became of her.
My sister and I have been working on the family history for a while now.If there's anything we could help you out with do let me know.