This is not meant to be unkind, but....I can't imagine that Cressida would ever be interested in maintaining a royal dress code. In pictures of Catherine from way back (except for the-ahem-sheer one), she's always been stylishly and suitably dressed for a future member of the BRF. I've never seen Cressida look pristinely well-clad- she is usually a little disheveled and disorganized in her appearance, not unusual in a young woman. Even at weddings...
Would she be willing to change her whole style and demeanor in order to be Harry's wife? (Not that Harry is always well-dressed, but when the occasion demands, he looks spiffy). If their courtship is thought of as a job interview, she hasn't done very well. Many girls might think they would like being a princess, but this girl doesn't look as though that's what she wants.
Of course, he may be like George Clooney and announce his engagement to the astonishment of us all!