I do see your point though, now that I reread your post, about Wentworth speaking too warmly to Louisa on the walk,

"My first wish for all whom I am interested in, is that they should be firm. If Louisa Musgrove would be beautiful and happy in her November of life, she will cherish all her present powers of mind.”

You COULD construe November of life, as a reference to marraige, especially when combined with that "all whom I am interested in" comment. He is very, very careless. It's especially maddening, because, again, I think he isn't doing it on purpose, and he's letting his feelings for Anne make him say these things. It's almost as if he's desparately trying to convince himself that Louisa is better than Anne and that Louisa is the girl to be preferred over Henrietta. That he's saying things aloud, infront of Louisa is just incredibly irresponsible, now that I think of it.

Also in the text there is the note, "he continued, with playful solemnity" that shows that he is not entirely serious during that speech. I don't think Louisa necessarily gets that.

I also have noted the unchaperoned walk in the morning in Lyme. I think that we don't know the circumstances of how that came about. Since Anne and Henrietta, their natural chaperones, had already gone off together, I guess they sort of fell in together. I don't see their unchaperoned walk as really being anything other than innocent, because Louisa is so quick to drop the idea to go to the shops. If either of them were keen to be alone together, I assume they would walk in two pairs and deliberately drop behind the others or outstrip them.