I don't necessarily accept that Queen Letizia didn't attend solely because of Marie-Chantal as they were together not so long ago at Prince Philip's service of Thanksgiving in London.
Princess Ingrid-Alexandra's 18th birthday gala, as glamorous as it might have been, is not an event to be attended by sovereign monarchs. Unsurprisingly, the royal couple of Sweden, the grand ducal couple of Luxembourg, the Queen of Denmark, and the King of the Belgians did not attend either. And there were no representatives of the British royal family. The crown princely couples of Denmark and Sweden were present because Frederik and Victoria are I-A's godparents and are also personal close friends of Haakon and MM.
I believe King Felipe VI also attended mostly as a godfather (otherwise I wouldn't expect the King of Spain to attend such a low-level event). It was a little bit odd, however, once Felipe had confirmed his presence, that his wife would not accompany him. I didn't quite get Mathilde attending alone either when her husband as expected did not, but that is not the first time (Mathilde was also solo at Carl Philip's wedding for example).