"B was a threat to Wentworth as Mr. Collins was a threat to Darcy."

Mr. Collins is a deliberately ridiculous charicature of a buffon. Benwick is more complex than that. I think Benwick teeters on the edge of being ridiculous at times, but Anne's steadfast defense of him at the end of the novel, show that he is meant to be taken seriously. Perhaps you are like the Admiral and find him a little too piano? Ah well, to each his own.

As Mia pointed out, the fact that Wentworth assumed that Benwick wouldn't be romantically interested in Anne because he was still grieving for Fanny Harville is proved by his shock at his attachment to Louisa. However, I still think it was possible that Wentworth was jealous of the intellectual connection and the possibility that Benwick would get to spend time with her at Uppercross when he could not.