Re:
I think that this is an obvious example of how the press interfere in the Royal Family's personal affairs and turn simple things into huge stories.
The Duchess of Cornwall chose to be known as such, and so she ranks as a Royal Duchess. She'd come before the Duchess of Argyll because she's a Royal Duchess but she'll come before the Duchess of Gloucester because she's the senior Royal Duchess.
A few people have said the Duchess isn't Princess of Wales. She is. The fact that she's asked to be addressed by a courtesy title doesn't mean that she isn't Princess of Wales. Diana is the late Princess of Wales, and Camilla is now the Princess of Wales but that in itself was a mistake.
Diana was titled as Diana, Princess of Wales before she died. She was not an HRH because she divorced the Prince of Wales not because the Queen took it from her. The Duchess of York also lost her HRH when she divorced Prince Andrew. It's standard procedure for a Royal Divorce. Diana's official title was, Diana, Princess of Wales.
As a butler, I've asked a few colleagues as to how Diana would have been addressed. She was Princess of Wales but she was not an HRH and she would have been addressed as M'Lady. The reason being that she was Lady Diana Spencer.
Diana would indeed have to curtsey to Princess Michael of Kent in theory but bowing and curtseying is no longer expected. It's nice if people do and 95% do, but it's a personal choice. Diana could have made a personal choice not to curtsey but that would be childish.
The popular press and some members on the Royal Forums and other message boards call Diana, 'Princess Diana'. This is totally incorrect because Diana never was Princess Diana. She was HRH The Princess of Wales when married and Diana, Princess of Wales thereafter. However, if she had been known as Princess ____________, she would have been Princess Charles. Females who marry into the Royal Family become Princess and take their husbands first name, hence Princess Michael. Before the death of the Duke of Gloucester (Prince Henry), the present Duke and Duchess were known as Prince and Princess Richard.
As to precedence, if Diana was alive today, then we'd be in a very different situation. She'd have been Diana, Princess of Wales until Prince Charles married Camilla, when Diana would probably be given a title (probably Duchess) and would be placed behind Camilla as the new Princess of Wales.
I don't think the Duchess of Cornwall will mind her rank because she understands that it isn't a slur but the ways of court. However, I mind very much that my future Queen is ranked behind the Princess Royal (who I have 100% support and respect for). Whether she chooses to be called Queen or not, or Princess of Wales or not, she is both and should be respected as such and take her place in precedence as such.