A possible theory about the letter (bear in mind this is all speculation at this stage) that AN will probably be presenting is that Meghan wanted to get her side of the story across to the press, but as she complains about in the latest court filings, was being told by the palace press offices that they would not comment. So she wrote a carefully drafted letter to her father telling him how devastating his actions were and advising him not to talk to the press. It is possible that she hoped he would take the letter straight to the press, who would publish it, and she'd get her complaints out without breaching the "no comment" rule. As Omid Scobie, her unofficial spokesman later tweeted, she knew her dad well and had an inkling that he would make the letter public, so much of it was written with the public in mind. It was also, of course carefully handwritten in her beautiful calligraphy, rather than sent by text and email, when her court papers show that she and Harry were generally communicating with her father by text. She also says she showed it to the press office, and sent a copy to her US lawyer (although they weren't allowed to read it....so...why do that?). It seems that this was a letter, then, of some significance.
But - Thomas Markle doesn't do what she expected and hangs onto the letter. Several months pass. Meghan's position is still not out there. Then Meghan's five friends talk to People Magazine, remember the existence of the private letter and spill about it. Funny that. People publishes its piece, stating that the letter was an impassioned plea to her father. Now Thomas, probably needled by this, finally decides to give the letter to the press. They print extracts of it, and Meghan sues them for breach of her copyright and privacy.
She can definitely expect to have it put to her by the AN's legal team that this was all an elaborate set up to circumvent the palace's "no comment" position, and that what she wanted all along was her letter in the public domain, but with plausible deniability. It will be up to the judge to decide what he considers the evidence to support, however