What if perhaps the maid who had committed the crime did it because they wanted to? Maybe there was a reason that Lady Melissa just didn't know about?
Maybe there was an explanation why the maid would commit a crime against her? Like maybe for revenge?
As cliche as it sounded, maybe a maid committed the crime to somehow make Lady Melissa pay for what she had done in the past?
Lady Melissa was a very cold and blunt child when she was younger and maybe that could've been a reason why?
But if a person was trying to bring Lady Melissa down then why steal the letters? What good would they do?
Maybe this was something bigger, this wasn't about stealing letters, and Lady Melissa was seeing this thing completely wrong.
Or, there was always the other option that Lady Melissa was seeing things completely wrongly and that she was overthinking too much.