By noun, I mean as opposed to phrase; i.e., unless I'm missing something this does not seem to touch particularly on why "man of science" [/ "men of science"] was not thought to be acceptable, given that it was apparently used about twice as frequently as "natural philosopher" in the surviving books of the time.
I also just noticed that in the 1831 letter, Whewell seems to have been sufficiently impressed by Somerville's book as to write her a poem about it. Such things were presumably more common at the time given that this was the Romantic era, but nonetheless would seem to sugesst that it made an impression on him.
It's pretty clear from all the rejected examples listed they're looking for specifically a one word noun (note they specifically don't mention "natural philosopher" either! Only philosopher). And it's moreover pretty clear that this is a group effort to come up with a word, not just something done by Whewell (he merely contributed the word "scientist").
Given the fact that Whewell uses the word "gentlemen" here, that other people are all doing the same task, and the fact that Whewell outright professes his reasoning for coming up with the word scientist, trying to pin this to Somerville just because the passage happens to be part of a review of her book is really really really stretching it.
As far as I can tell you're relying really really heavily on the coincidence that Whewell has an immense respect for Somerville and the fact that other people happened to use the term "man of science" to argue that he coined "scientist" after Somerville (or at the very least sow doubt in his own stated intentions). And that feels like you're taking Whewell way out of context.
I also just noticed that in the 1831 letter, Whewell seems to have been sufficiently impressed by Somerville's book as to write her a poem about it. Such things were presumably more common at the time given that this was the Romantic era, but nonetheless would seem to sugesst that it made an impression on him.