On the one hand, it's a coronation, it's the Easter season and it's a major historical event. It's ultimately their duty to be there, and a silly spat that will get cleaned up in few years shouldn't stop them. On the other hand, it's really all about Charles, and I don't see how unrepentant children serve him in this capacity. Nobody likes The Princess of Canada, so it's really like Andrew. If they want to come, have a private chapel service with them and Andrew where they swear fealty and take pictures. Unless, the Americans closes the investigation, and Harry publically apologies, of course.