I've been having fun following all sorts of leads, and I can tell you that what appear to be two different women in that French letter are really one and the same: Hortense Lepaute, wife of the royal clockmaker, Mme Nicole Reine Lepeaute, celebrated mathematician all one person. Her full name was Nicole-Reine Etable de la Briere Hortense Lepaute; I haven't actually been able to establish whether she used her middle name "Hortense" publicly at the time or not. She was more than a mathematician:
(From a "Women in Astronomy" site)
NICOLE-REINE LEPAUTE: Nicole-Reine was born in
France in 1723. Not much is known of her childhood. She married Jean-Andre Lepaute (1720-89) who was the royal clockmaker. Nicole studied the oscillations of pendulums of different lengths, and her results were published by her husband in his Traite D'horlogerie in 1755.
She was hired by J.J. Lalande, the director
of the Paris Observatory, to help Clairaut determine the nature of the gravitational attraction of
Jupiter and Saturn on Halley's Comet and calculate the exact time of its return in 1759. Lalande gave Nicole full credit for her work. She went on to
calculate the path of the 1764 eclipse of the sun
for all of Europe and the chart was published by the French government. for 15 years, from 1759 to 1774, she helped Lalande with an almanac for use by
astronomers and navigators, published by the
Academie des Sciences, and then from 1774 to 1783, she worked ont he 7th and 8th volumes of the
Ephemeris, calculating the positions of the sun,
moon, and planets covering the decade from 1784 to
1794.She dies a quiet death in 1788 and a crater
on the moon is named after her.
I think that French post you gave probably has the gist of the case right. Some flower was named by Commerson or someone else in honor of Mme. Lepaute; that post has "peautia caelestina" and I've seen it as "Lepautia caelestina". Celestial, get it? (I can't find any official record of the name.) Then the same person or someone else decided that the Lepautia plant was really a Hortensia, an already-existing genus, and so changed its name. That sort of realization and name change happens fairly often. Yet it is just tricky enough that it might have confused the OED.
The French botanist Commerson's got his own dramatic story he took his girlfriend Jeanne along on the big 1766 expedition run by Bougainville. She disguised herself as a man and worked on the crew, and the deception wasn't discovered until Tahiti. She and Commerson left the ship at Mauritius (near Madagascar), where Commerson still did what he could for the natural sciences until his death five years later. So, he never got back to France.
"Either way, it seems like an awfully convenient name for ... or belonging to a garden' (f. hortus 'garden')."
Precisely why Commerson created the flower name "Hortensia" is still unknown to me. Probably the "garden" meaning. But maybe in honor of someone. Or both.
Best Donna Richoux