I agree with that point! It would be just like W to blame his new wife for something he was ashamed to do to M. Again, his purpose to come to see M was because he thought she was dying and wanted to revel in the idea that she died due to love for him. And then that didn't happen, so he wanted to justify his reason for coming by talking to Elinor and absolving himself of his guilt. He was not going to mention the letter at all, until Elinor prompted him, so I do think it could have gone either way! He could have written it, with Sophia's help. And by admitting that she composed it, this showed more of W's impotency... which isn't particularly flattering to W. But he was at that moment still trying absolve himself of his thoughtlessness of M, guilt of his actions, and profess a continued undying love for M despite his marriage to another woman.