NavList:

Robert, thank you. That confirms what I had found so far and fills in some key details.

You wrote:
"Bode named it 'gemeinen Dämmerung' or 'bürgerlichen Dämmerung' and defined it as the moment when the twilight arc (the earth's shadow on the atmosphere) passed through the zenith, thus conveniently marking the time when artificial lighting was needed for indoor activities. Bode adopted Lambert's value of nearly 6.5 degrees"

I am sure you meant that light was needed for 'outdoor' activities. What you've described here is exactly what's written in H. H. Kimball's article, publiished in 1916. Kimball notes that this process of trying to spot when the twilight arc crosses the zenith was nearly impossible in practice (I've never detected the twilight arc anywhere above 20° altitude!) which was why it made sense to pick some arbitrary altitude below the horizon. He noted that there were various choices in use at the time, but he chose 6°. His choice seems to have been influential; many later publications reference his. 

You added:
"Nautical twilight was first introduced in 1936 by Leslie John Comrie"

That date certainly fits the evidence. Have you been able to find actual documentation on Comrie as the source? I understand that he was director of the almanac office at this time, but did Comrie personally introduce this concept? There was an awful lot going on at this time --the transition to the New Navigation was in full swing, tables with GHA were just being introduced, air almanacs were rapidly expanding-- and though I am confident that you're right that HMNAO was the first almanac office to introduce nautical twilight since the earliest references to nautical twilight around 1940 attribute it to British almanacs, it seems to me that there may be a little more to it.

You noted that nautical twilight was introduced "as a convenient means of dividing the interval between the end/begin of civil and astronomical twilight in two nearly equal parts. Regular tabulations of the times of nautical twilight first appeared in the 1937 edition of _The Nautical Almanac and Astronomical Ephemeris_ and its abridged version for navigators."

And for modern celestial navigators and navigation enthusiasts, I think it's important to remember that this is literally all there is to it. There's no big science to it. As far as we know, the end of nautical twilight was not determined by any careful series of observations of the sea horizon trying to determine exactly when the horizon disappears, although there may have been some rough experiments along those lines. That 12° limit is just arbitrary. It splits the difference between civil and astronomical twilight, and that's all it means.

-FER