So it seems likely that Betsy's parents are John Scott and Isabella McNaughton, However I cannot find evidence of a Marriage or nail down the births of either John or Isabella.
Missing records are no unusual in the early 19th century. If you can't find the marriage on Scotland's People there's a high likelihood that no record of it has survived.
Have you looked at the originals of the baptisms of Betty and Mary on Scotland's People? What (if anything) more do they tell you?
It may be possible that the John Scott was the son of Walter Scott Esq.
Why do you think so? What evidence is there for such a connection?
As for George Abbot. His father was William Abbot Carver & Guilder(I think) and Mother looks like Jessie Abbot but I can't be sure and I cant make out her maiden name.
Can you post an extract from the certificate that shows George's parents' names and see if we can decipher it?
I see that George Abbot and Elizabeth Scott had a son Thomas Alexander Abbot, born in Dundee in 1855. You are in luck! 1855 certificates contain far more information than earlier baptism or later birth certificates, so go to Scotland's People and get a copy of it.
I see from the index to the census that there were several other children - Betsy, George, Jane, Charles, Robert and Mary. There may also have been a John who died in infancy. So it seems that this family was not into getting the children's baptisms recorded. This may point to them belonging to a religious denomination whose records are not readily available, for example Episcopalian.
If there really was a connection to Sir Walter Scott you would expect them to name a son Walter.