Having read it, it's glaringly obvious (to me, at least) that the letter to her father was quite deliberately written in the hope that he would promptly release it to the media for publication.
It conveniently presents Meghan's version of events and responds to all the issues she felt unable to respond to directly, because of being muted by the RF.
Unfortunately for her, this plan failed when her demoralised father neglected to sell the letter to the press.
Months pass. Finbally, in frustration at being unable to get her point across publically, Meghan implements Plan B - which involves getting the five 'friends' to introduce the letter's contents into the equation, but crucially, implying that it was a kinder, more loving sort of letter than Thomas Markle would have perceived it to be.
Thomas Markle duly takes the bait and releases the letter to the press. Unfortunately for MM, it really doesn't have the desired affect - because she has completely misjudged how the public would react to hearing 'her side of the story'. This is familiar terrain - it has happened several more times with various leaks via H&M's contacts about their plans, thoughts and activities.
And so we end up in the extraordinary situation in which Meghan is effectively suing the Mail for publishing a letter that she fully intended them to publish all along.