As you will see with Eugine should she have children her title will die with her and not be passed on to children, much like what will happen with Harry's.
Not quite, the title 'Duke of York' is often given to the ruler's second son, but it wouldn't make sense to give it to Harry without giving his uncle a different title, even if they did that, Sarah Ferguson would still be 'Duchess of York' so you would end up with Andrew being Duke of somewhere to suit his ego, Harry being HRH the Duke of York, Meghan would be HRH The Duchess of York and Fergie would still be the duchess of York.
So, I doubt it will go to Harry but might go to Louis, his children (assuming William is King) would then be HRH Prince/ess something, of York
and after her divorce, Diana had the title of princess but was not an HRH.
Not quite, Diana was NEVER Princess Diana, she was HRH Diana, Princess of Wales.
It's a bit like the other thread running on Mrs Hisname His surname.
If William and Harry had not been given Duke titles then their spouses would be Princess William of Wales and Princess Henry of Wales. Well actually they are that, as well as Duchesses.
I think the loosing on not getting an HRH is the way the royals publicly show who they are pissed off with, Wallace Simpson didn't get an HRH.
When the then princess Elizabeth married she became HRH The Princess Elizabeth, Duchess of Edinburgh because she was already a princess in her own right.