The princess:
Princess Maria Laura's
official style will certainly continue to be HRH Princess Maria Laura, in the same way that her mother (since the succession changes of 1991) is officially styled as HRH Princess Astrid, Princess of Belgium.
https://www.monarchie.be/en/royal-family/princess-astrid
However, if Laura and William continue to live in Britain, I wonder if she will call herself Mrs. Isvy in private life, in accordance with British custom.
Her future husband:
William Isvy will not acquire any legal titles; Prince Amedeo's wife Lili Rosboch did not.
However, the Royal Palace currently styles Lili
by courtesy as Princess Elisabetta Rosboch von Wolkenstein, which is often shortened to Princess Elisabetta.
https://www.lesoir.be/246315/articl...dit-le-prince-amedeo-et-sa-femme-annoncent-la
https://www.theroyalforums.com/foru...elgian-royal-family-38975-17.html#post2186635
Will the Palace likewise give William Isvy the courtesy style of Prince William Isvy or Prince William after marriage? Probably not, as there is little to no modern precedent in the nobility for a man to be addressed with his spouse's title even as a courtesy, but unlike the nobility, the royal family has introduced gender-equal succession for the descendants of King Albert II.
The possible future children of the marriage:
Any children from the marriage will almost certainly take their surname from their father. It
remains exceedingly rare for a Belgian woman to transmit her surname to her children, and I believe it is even rarer amongst the nobility.
Any children also will not inherit their mother's HRH or territorial designation "of Belgium". The predicate HRH and the title Prince/ss
of Belgium are no longer hereditary, except for the children and grandchildren of the King or Crown Princess, under the Royal Decree of November 24, 2015.
https://www.ejustice.just.fgov.be/c..._date=2015-11-24&numac=2015021075&caller=list
However, Article 4 of the decree refers to "Princes and Princesses" who descend from King Leopold I but are
not Princes and Princesses "of Belgium". Prince Amedeo's children appear to fall under that category, as his daughter Anna Astrid was entered into her birth certificate as "Her Imperial and Royal Highness
Princess Anna Astrid Marie Archduchess of Austria-Este (Habsburg-Lorraine)" (emphasis added). Moreover, the Royal Palace has always given her the style of Princess Anna Astrid (rather than Archduchess Anna Astrid).
https://www.lesoir.be/53935/article...ur-les-princes-qui-ne-seront-plus-de-belgique
https://www.theroyalforums.com/foru...elgian-royal-family-38975-16.html#post2117594
But it is unclear if Princess Maria Laura's possible children would be styled in the same way, being children of a Princess instead of a Prince. Although Article 4 of the Royal Decree says nothing about "male line", titles of nobility such as Prince/ss are normally transmitted in the male line only.
https://diplomatie.belgium.be/en/services/Protocol/nobility_and_honorary_distinctions/nobility/faq