I think as there has been so much talk about how awful the royal family is, the need to escape, etc., and Harry and Meghan are wanting to do it all differently, they should use Markle. Archie Markle, Lilibet Markle and Harry can surely change his name to Harry Markle if he feels left out. Voila, problem solved!
I am pleased to direct your attention to the Queen's declaration made in April 1960, which amended one from 8 years before, 1952, concerning the name of the royal house that she and Phillip started.
"Now therefore I declare My Will and Pleasure that, while I and My Children shall continue to be styled and known as the House and Family of Windsor, My descendants other than descendants enjoying the style, title or attribute of Royal Highness and the titular dignity of Prince or Princess and female descendants who marry and their descendants shall bear the name of Mountbatten-Windsor.""
So you will find, dear @daretodenim that because Harry and the kids enjoy the titular dignity of Prince and Princess, they do not need a surname and if ever they did, that surname would be Mountbatten-Windsor.
Harry and the children would only ever be called Markle in your fervid imagination.