] Through an acquaintance with Mrs. Smith, she could go alone with Willoughby or without an acquaintance she could go when Mrs. Smith was out if there was a chaperone.

I think this is correct. People frequently went to view the estates of wealthy individuals whom they did not know, but generally went when the person was not in residence, as when Elizabeth Bennet visited Darcy's estate with her aunt and uncle. Of course Elizabeth knew Darcy, but it would not have been considered improper if she hadn't know him as long as she was accompanied by her aunt and uncle. Since Marianne didn't know Mrs. Smith, she could have waited until Mrs Smith was gone, and then visited the estate with Willoughby and another person, such as her sister. The entire operation seems strange to us because we live in a society where people address each other by first name the moment they are introduced (if anyone takes the time to introduce them, that is!).