I too had never doubted that Sophia wrote the letter, but all these posts really got me thinking.

Sophia was rich as could be, she could dump him and find someone else quickly. So why did she go through with the marriage anyway? Maybe just vanity, she didn't want the humiliation of calling off the wedding. She probably loved him or thought she did. Notice he said "She does not deserve your compassion. She knew I had no regard for her when we married. It's plausible that she may not have had a hint of his having no regard until she found those letters and lock of hair. Notice he makes no mention of whether she had regard for him.

Willoughby's words are telling...
In honest words, her money was necessary to me, and in a situation like mine,anything was to be done to prevent a rupture.

Rachel, I think that how you describe it happening seems very likely. She may have actually written it, but she probably based it on his story to her- something like "this silly girl inexplicably had a crush on me and sent me letters and a lock of her hair." I can imagine that she may have relented and decided to still marry him, but may have still been suspicious of him, and wanted to witness him writing the letter putting an end to the silly crush.

I agree that we don't know that she is a malicious sort of person. The words of the "infamous letter" don't seem in themselves malicious to me. What made the letter so horrible to Marianne and Elinor is that it was so opposite of the truth. That truth is what he most needed to conceal from Sophia to get her necessary money.