My conclusion is this:
Their marriage has been strained for years, due to Williams well known indiscretions. The circles are unpleasant places to be if everyone is talking about your dh and whispering pegging. They have never liked her anyway as she doesn’t come from an aristo family. She is not one of them.
Kate discovered she is unwell and requires surgery, William hasn’t stepped up and supported her at all, regardless of his public declarations and her parents have stepped in. Kate has finally had enough. KP panic with the crisis growing and buy time in hope of mediation.
The photo on Mother’s Day is published to give the public the impression they are a happy family. That blows up. Kate is forced to travel with William (despite the fact they aren’t talking) to quell growing rumours and speculation about Kate’s health and whereabouts. Kate turns her face. It’s a silent rebellion, but speaks volumes. Her wedding ring is missing and so is the engagement ring she treasured at the time.
The Royal family is in crisis.