I think the first question for a subject like this one (Was NB's #16 swale the inspiration for the swale in Macdonald's Biarritzes?) is whether or not the famous 12th hole at Golf de Biarritz (famous Chasm par 3 hole across the Bay of Biscay) had a prominent swale in it.
If Macdonald saw that hole which he obviously did because he mentioned it in his well-known article in Outing Magazine in 1906 about an ideal course (published before he bought the land for NGLA) I think we'd have to assume it was the architectural product of the Dunns and apparently from 1888.
I have never seen anyone answer that question with authority or any real textual, drawing or photographic documentation. I have seen an old photo or drawing of that hole but I believe it was from the tee and it is basically impossible to make out what was on the green or green-end as far as a prominent swale is concerned.
And even though he was certainly familiar with North Berwick I'm not aware that Macdonald ever mentioned a prominent swale in #16 or that he used that idea from it anywhere. Is anyone aware of that?
This is how Macdonald described in 1906 his architectural thinking regarding the 12th hole at Biarritz:
"210 yards. Suggested by the 12th Biarritz, making sharp hogback in middle of course. Stop 30 yards from hole bunkered to the right of the green and good low ground to the left of plateau green."