My cousin is very thankful for your advice (as am I). We got some material from Scotlandspeople and now know that the 1773 christening is the right one. It not only fits the death age, as you pointed out, but also nominates that she is the daughter of "James Donaldson Baron Officer", which matches his description on her death record.
The 1780 James Donaldson "death" record at Scone turned out to be some sort of accounting record which mentions him. We also thought we might have had the death of John Gellatly's father James Gellatly at Scone in 1785 but it was also a book of accounts. The same thing happened with a 1776 "death record" for John Blyth (b 1701) at Kettle. I'm not sure why these are listed as death/burial records.
We now have the christening of Margaret's mother Christian (Christane) Blyth(e) at Newburgh, Fife, in 1840, and the marriage of her parents John Blyth and Margaret Anderson there in 1821. Apparently Christian was the last of nine children, which explains why she was born 19 years after the marriage. And lots of other interesting things.
We hunted without success for the deaths of James Donaldson and Christian Blyth(e) (Donaldson) in Perth and Fife in the period from 1779 (when David was conceived) to 1855 (when they are listed as deceased on Margaret's death record) -- casting our net as widely as we thought reasonable. Interestingly, we also did not find a death at Scone for James Donaldson in 1795, as was indicated in those Ancestry trees.
We have had a lot of success really, and have used 66 credits on our little spree, with 24 still up our sleeve. I am sure we will find plenty of constructive ways to use them!
Again, thanks for all your help.
Geoff (and Lisbeth).