I Think the "mere nothing" comment relates to the fact that she isn't in the same level of distress she was when he first came into their circle. Over time she's gotten used to it. I don't think she is even attempting to think to herself that she is over FW.

The fact that she befriends Benwick has nothing to do with Wentworth at all, except that the situation with him has made her more sensitive to someone in Benwick's position. Benwick also has similar taste in reading and most flatteringly, he appears to want and listen to her advice. No one else, yet in the novel has showed such an interest in Anne's brain.

There's a blossoming that happens to Anne in Lyme. The narrator acredits the sea air, but I think it's the male attention she receives, first from Benwick, then from Cousin Elliot, and finally from Wentworth himself. The latter of which is the most noted by and important to Anne.