What are you asking exactly?
Meghan will have to travel on a U.S. passport with U.S identification until she has British citizenship. Her name can remain Markle or she could change it to Mountbatten-Windsor once married if she chooses. Duchess whatever would not appear on official U.S. identification. British laws/rules have no bearing on U.S. documents.
I am asking what the rule is for name changes in Britain upon marriage (although I assume it doesn't apply to her as long as she is a US citizen). In the US women have to actively change their surname after marriage if they choose to do so (it isn't automatic), some countries automatically 'add' the husband's surname to the wife's surname (or 'change it'); others countries ask the couple to indicate beforehand what the name will be upon marriage (in: becoming effective at the same time the marriage is made official). So, that was what I was wondering...
Meghan cannot have two different legal names. She either is Rachel Meghan Markle or Rachel Meghan Mountbatten-Windsor (or something else); and it seems that as it has been communicated that she remains an American citizen that it will depend on her whether she goes to whatever place you have to go to (she will know as she already changed her name twice) to either keep it Markle or change it to Mountbatten-Windsor (most likely, as that is to be used if not titled).
if her U.K. documents say HRH Rachel Meghan, Duchess of Sussex, then the closest match will be Rachel Meghan Sussex. I believe they all use territorial designation as last name unless there is a specific reason why she can’t. She can in US even if her title isn’t officially recognized. Again, technically Harry isn’t Harry Mountbatten-Windsor as he is excluded in the LP allowed the Queen and DoE’s descendents to use Mountbatten-Windsor as he does have a princely title.
Given that Meghan's title wouldn't be recognized in the US, I would argue that in that case Mountbatten-Windsor would be the appropriate surname as that is exactly the situation in which that surname is to be used (if there 'is' no title). She won't be 'Princess Meghan of Sussex', but Her Royal Highness The Duchess of Sussex (or Clarence or ...)', so the combination of her first name with Sussex would be inappropriate from my point of few (as it is well known that that combination is used for divorced former duchesses).
not disagreeing with you, but just to point out another possibility. She could change her legal last name to ducal (Sussex or whatever she becomes the duchess of) if she wants in US if she changes her last name. My understanding is that Harry and William doesn’t use Mountbatten-Windsor in U.K. as that only applies to descendents without a princely title. In their U.K. documents, I don’t believe they use a last name. Or they used Wales as they were/are Princes William and Harry of Wales. George also uses Cambridge as his last name.
I don't think George uses Cambridge as his last name on legal documents - but yes, just like William and Harry did with Wales and Beatrice and Eugenie with York, they use the territorial designation in daily life if they don't want to use their full title.
Could she indeed just pick whatever she likes as a surname or does it need to be related to something. As in, could she decide to take the surname 'princess of the United Kingdom'?
The logic flaw IMHO is that the BBC, the British press, and the politicians want Meghan to go through the normal naturalization process as any "ordinary person" when, in fact, she won't be an "ordinary person", but rather a princess of the United Kingdom. As Somebody said, I don't see how a princess of the United Kingdom, who has official representation duties on behalf of the British monarch, may not be a British citizen.
Again, the Danes and the Dutch dealt with a similar issue very quickly in the cases of Mary and Máxima. I understand immigration is a hot political issue in the UK these days, but I don't see the reason why Meghan's fast tracking her naturalization process has become such a big "no-no".
And if the argument would be that Mary and Máxima (and Stephanie) married the heirs; Marie was also fast-tracked (as was Alexandra I believe) and married a 'second son'.
I wonder whether they really thought it through, as there are representation issues (in my view), issues with children who might be born with the American nationality, tax-issues, etc.
Does anyone know how the citizenship issue was dealt with for Autumn (relatively recently) and for Birgitte (and Marie-Christine) in the past?
If, hypothetically, she wanted to change her name legally in the US, wouldn't it make more sense to change it to Rachel Meghan Mountbatten-Windsor rather than Rachel Meghan Sussex or anything like that ? Matching her UK and US documents will never be possible as her UK documents will be issued in the name of HRH Rachel Meghan, The Duchess of Sussex and that is a name which, I assume, she cannot use in the US. She would probably be better off then sticking with Rachel Meghan Markle, at least until her naturalization process in the UK is complete. After that, I assume that she will renounce her US citizenship and the question of how her name will appear on her US documents will no longer apply.
EDIT: Doing a quick Google search, I found copies of Prince Charles and Diana's passports in
the following link . I assume Harry's and Meghan's would be similar, right ?
Thanks! That's helpful. And might be an argument to keep it at 'Rachel Meghan Markle' as for Diana her birth name 'née: Lady Diana Frances Spencer' was included. So, there would be a match between her US and UK documents.