Not sure if this has already been asked but if the letter was written to Thomas Markle surely the letter would be his to do what he wants with it. If that is the case then Meghan has no case as it's not her letter
That's the difference between copyright and ownership.
He is the owner of the letter as he was the intended recipient and he possesses it.
But the writer of the letter can remain the copyright holder (there's may be legal points around whether that is always the case) and so it should not be reproduced without the copyright holder's consent - other than small excepts for (fair) comment.
So the case will look at who owns the copyright, whether previously published accounts were authorised, whether TM's actions were reasonable (in correcting unfair comment) and whether AN should have published.
MM launched the case against AN, as she could hardly sue her own father, but I think the reasonableness of his actions may well be a feature.
Her case against them will be scrutinised, so to that ecpxtent she is fours on trial. But the way she put it is snappier than 'claimant' and feeds her wider (and already partially struck out) narrative.