I think the clue is Anne's thought to herself, "of course, they fell in love over poetry." She has already seen the first glimmerings of his rallying from Fanny's death, over a brief discussion of poetry with herself. He has had countless hours with Louisa, in a helpless and "interesting state" to fall in love over poetry.

I don't believe he was in love with Anne. I think he only wanted to meet with her again to continue their discussion. Perhaps his spending time with Louisa interferred with that visit, until the point that he realized that he didn't need to make it. You can call this switching affections, but since we have no evidence that he had any affection for Anne other than friendship, at that point, I'm not convinced.

it strikes me that what LR thought about Captain Wentworth is actually true of CB: "The man who...had seemed to understand somewhat of the value of an Anne Elliot, should,...afterwards, be charmed by a Louisa Musgrove."

Oh, that is true indoubtably. What a wonderful irony that you've pointed out, which I'd never noticed. I defend Benwick only in that he is a decent enough fellow, with a heart worth having not to say that he has the best possible taste or self-awareness. As Wentworth says, he is a thinking and feeling man, but this tendency to feel deeply means that his emotions are rather unstable. He seems to fly up and down in a way that is less than desirable. Wentworth is disturbed by it, but Anne is not, perhaps because she is so grateful to him.

I can't help but like Benwick because Anne does, and because he has been extraordinarily useful to her in his brief aquaintance. Firstly it was his small attentions to her at Lyme that began the restoration of "something like Anne Elliot" and secondly, he has taken Louisa Musgrove off Wentworth's hands. What a good friend!