Chrissy, you're seeing a lot more consistency than I am in all this.
1. Wallis married an HRH and remained plain old Wallis despite the fact that the LP granting the Dukedom of Windsor specified the dignity of a son of the sovereign for Edward.
2. Sarah became HRH when she married Andrew, lost it along with membership in the RF when they divorced.
3. Diana became HRH and lost it when she divorced Charles, but remained a member of the RF.
I never mentioned Wallis and wouldn't do so as times change and her situation was different in that she married an abdicated king who had caused a lot of angst within his family, the government and the Empire at the time.
Diana was given a one off special treatment as she was the mother of the future king and for no other reason. Had she had no children she would have been treated the same as Sarah.
Another way of looking at this is how would Sarah be treated if William and Harry both died without legitimate issue of their own meaning that Sarah would be the mother of the future Queen. I am sure in that situation that she would be given the status offered to Diana for that simple reason.
In general terms Sarah and Diana were treated the same - loss of HRH on divorce but Diana was given a separate treatment due to being William's mother only.
Looking at that, I cannot come to the conclusion that Sarah and Diana gained the HRH as a consequence of marriage to an HRH, because Wallis did not get HRH.
Why else would Diana and Sarah get the HRH. When they entered the Abbey/Cathedral they weren't HRH but when they left they were. What had happened in between - they got married. The same with the Duchesses of Gloucester and Kent - got the HRH when they got married, as did the Queen Mum and the present Duke of Gloucesters mother (the Duke of Kent's mother already had it in her own right as a Princess of Greece).
In other words Wallis is the only exception and not the rule.
The rule is very clear - on marriage to a male HRH the lady gets the HRH.
Nor can I conclude that HRH denotes membership in the RF because Diana lost the HRH and stayed in the RF. You clearly can be a member of the RF and not be HRH.
Of course you can or do you say that the children of Princess Anne aren't members of the Royal Family?
All I can conclude is that the sovereign bestows and takes away HRH as she sees fit.
No - the monarch has general guidelines which are used except in very exceptional circumstances - of which there have been two in the last century.
Take another look at the number of sons/grandsons of British monarchs (those with the HRH) in the 20th century since the 1917 LPs who married non royals and tell me how many of those wives did not get the HRH on marriage.
6 sons, 3 grandsons - of whom ONE wife didn't get the HRH on marriage. The sons are George V sons - David, Bertie and Henry (George's already was HRH), and the grandsons are Richard, Edward, Michael, Charles, Andrew and Edward. In other words the exceptional circumstances surrounding Edward's abdication have to be considered in understanding why she was treated differently to the wives of his brothers, nephews and grandnephews who wives all got the HRH on marriage.
They are not properly termed courtesy titles; they function more like the GCVO or KG. Diana being the mother of a future king, and Wallis being hated, are reasons that explain why the Queen did what she did, but they are not Constitutional principles that explain how this process works. I'm ok with everything the Queen did, I just wish she'd done it in a way that establishes clear and consistent principles for the future.
The rules are consistent - marry an HRH Prince and you get HRH - unless you do something really bad such as cause an abdication.
Divorce an HRH Prince and lose the HRH unless you are the mother of the future monarch.
But I believe HRH/princes have divorced before, not the heir but HRH/princes nonetheless. What happened in those instances?
Since the 1917 LPs this hasn't happened except for Diana and Sarah so earlier examples would have been covered by different conventions.
PS - edit to add, Princess Michael doesn't need princely dignity to be Princess Christine, all she needs is a peerage in her own right. (I think authorities split on the issue though.)
No she needs the LPs to give her a title of Princess. e.g. the wife of most titled people in Britain are not entitled to call themselves Princess xxx. The title is of a lower rank.
To be Princess Christine she would need to be created it and the Queen want to that. To be given the title of Duchess in her own right wouldn't let her be called Princess Christine but Christine, Duchess of xxx. The same with any other peerage in her own right.
I don't believe the Queen ever had the power to bestow princely dignity on Phillip anyway.
On what grounds to you say that?
The Queen is the Fount of all Honours and she could give him that title if she so chose. The only title she couldn't give him is King as Queen Victoria had already received advice to the effect that only the Parliament could create a King.