I don't doubt that you have all these original documents, and I agree that the dates match neatly and that spelling is irrelevant - I've also seen other variants beside the ones you have found.
What worries me is how you can be sure, for instance, that the James Mitchell who was born in 1584 in Dunfermline is the one who fathered James in Elgin in 1609? Or how to be sure that every one of the Mitchells who fathered a son in Montrose in the 1700s was born in Montrose and not a different person who had moved in from another parish? That's why you need to find independent evidence to prove that they were who they were.
Mitchell, incidentally, was the 19th commonest surname in Scotland when the Registrar General did a survey in 1990. This makes it much more difficult to be confident of each succeeding generation
As GR2 has said, the further back you go, the fewer people are actually in the records. This means that unless you have some independent proof, from sasines or wills or some other source, you cannot assume that a likely candidate is the right one.
When I started out with my tree, I found someone with exactly the right name to be my 2ggmother, born in exactly the right parish in exactly the right year, so I assumed she was my 2ggm and spent a lot of time working on her family. Then I got the death certificate of my 2ggm and discovered that she was an entirely different person whose baptism record had not survived.
I happen to know, because I looked it up for a similar discussion quite recently, that just over a fifth of people in my tree born in Scotland between 1780 and 1800 have no surviving baptism record. By the time you get back to 1600, there are many parishes with no surviving records at all. See
https://www.nrscotland.gov.uk/research/guides/old-parish-registers/list-of-old-parish-registers#List%20of%20OPRs for details of coverage by parish.
If you look at Elgin, you will see that there is a gap in the baptism records between 1679 and 1705. So it is perfectly possible that the father of James, born in Elgin in 1609, was born in Elgin between 1679 and 1705 and is unrecorded.
As I say, I am sorry to be a wet blanket, but baptism and marriage records alone are not sufficient to prove a line of descent.
I take it you have the Tasmanian death certificate for Robert (1809-1880) naming his parents as Robert Mitchell and Helen Christison?