I don't mean to step on anyone's toes but I feel compelled to set the record straight: Crown Princess is Crown Princess, born royal or not! She is elevated to the same status as her husband. There is no protocol that dictates one has to curtsy to another. At times, now or when they are both queens, Vicoria's birthright may come to the fore in seating and precedence when the two appear together at a function. When they are with their spouses, that changes, if it needs to.
I can't vouch for Sonja curtseying to Margrethe but as far as Mette-Marit goes she might have been caught up in the moment and just curtsied once too many times.However, you all overlook another possibility: she might have been trying to get under the hat for a kiss on the cheek! Besides, in a curtsy, a lady steps back with her right leg; Mette-Marit extended her left leg and to me she looks like she's going for the right cheek (the one you usually kiss).
As far as I know, there is no such rule and I have never seen it applied. There's also nothing to stop her making a goodwill gesture...?