I think that degree of protocol is used for the Queen and Prince Philip and that's it. When Charles is King Camilla will be half a step behind him on state occasions. This is no different from the PM's wife at a state or formal occasion. Two Heads of State meet, shake hands and then introduce their spouses. It is courtesy not what century we are living in.
A perfect example of the expectations of the heir and spare is that of the Crown Prince of Denmark, Frederik and his brother Joachim. When arriving at formal occasions, first out of the car and onto the carpet is Marie followed by Joachim who gets out the other side and walks round to join her. Then comes Frederik who gets out to the car and steps onto the carpet followed by Mary who gets out of the car and walks around to join him. The comes the Queen and Consort and the same format is followed.
The spare is third in importance so he can decide who sits where in the car, etc. However, if he is carrying out a Royal Duty he'll still get out of the car second, but will greet the dignitary and introduce Marie which is, of course, only good manners.
Harry is fourth in importance and since State Dinners are not televised, who knows? Actually, that is a change they can make . . . televise the arrivals and parts of State Dinners or important occasions, then the UK will know what their annual 65p buys them.