I lurk usually but am also intrigued by the 'why have they come back?' question.
I think it's simply because they have made a dreadful mistake. Or at least H has. The euphoria of 'escaping' his family, friends, purpose and means of support was never going to last. Going back to a celebrity life in California was presumably what M preferred to do but she wasn't able to make that happen and is now dragged back into H's attempt to recreate what he had before. But what they did in their attempts to be Californian celebrities now means they can't have what they had before. And the spiral continues.
It's been said many times before but they could have had a very nice life as minor royals, working or not. Or they could have engaged people who actually know PR, branding, marketing, business and media, taken their advice and had a very nice life in California as the faces of luxury businesses, good causes and harmless content about aristocratic life.
They are very ordinary - and self-absorbed- people with no special talents but instead of choosing one of these very nice lives, they listened to no-one, and embarked on this foolish journey until they ran out of road. They can't admit they made a terrible mistake, probably not even to themselves. So here they are, and I don't envy them.
Neither do I envy the various authorities who have to judge what kind of damage might be done to the country's reputation by these two as they spiral on.