As for the
Duke of Marlborough title,it was him who wanted to be Prince of the Holy Roman Empire and approached the Emperor through his envoy Count Wratislaw von Mitrowitz...The Emperor was not at all thrilled and because he felt obliged to the great soldier to give him at least an honorary title he asked Queen Anne for the formal approval which was "duly given",according to the letters of Count Wratislaw to the Emperor from the 22.August 1704.
But,the Duke did not want an "empty title",but an effective one and that's when they started to search for an empty land which was also Principality-just for him.They found Mindelheim which was bought by the Elector of Bavaria in the 17th century,but which Bavaria lost in recent wars. They awarded it to John Chruchill,now the Prince von Mindelheim,who again lost his Principality to Bavaria without compensation some ten years later. But,this time,Emperor Charles VI felt obliged to his German dominated Princes,who all,according to the notes from the Memoirs of the Duke of Marlborough,looked upon the Duke as "only a small country gentleman,from a country at the very edge of Europe" and wrote an apologetic letter to him,but did not do anything to restore his seat and vote in Imperial Diet. Of course,at that time Marlborough was not the force in Europe he used to be and did not dare to militarily defend his rights as an Imperial Prince against Bavaria and therefore had to settle for anything Empire had offered to him as a sort of consolation.
He later protested his male law succession for an Imperial title through Prince von Anhalt-Desau,but to no avail...and the line died out with his daughter,Princess Mary in 1751.
You are right,his descendants could have become mediatized,if,for example,the female-line succession was officially permitted and the family after acquired a sovereign part of Imperial land until the official mediatization...This only could have made Lady Diana "equal" as the family would fall into II part of Gotha-Almanach ,but even without requiring sovereign land if the family were allowed to use female succession of the title,Diana could have been Princess in her own right...even though,not equal!
But,again,it is a question which of the sisters would be "the one" who would transmit the imperial title if female succession have been permitted...
Maybe it could have been settled similarly to the Bentinck case,where an elder son succeeded English titles(Earl and Duke of Portland) and the issue of the younger brother were the Imperial Counts-reichsgrafen of Bentinck-Aldenburg?
And the family separated until Timothy Bentinck became Duke of Portland,whose great-grandfather Henry Charles deferred his rights as the Head of the Bentinck-Aldenburg family in favor of his brothers in 1874 upon marrying Harriet Eliza Cathcart McKerrell,a woman whose family McKerrels of the Hillhouse belonged to the old landed gentry lairds dating back from the 15th century,but in the eyes of the House of Bentinck she wasn't good enough...
Here is a picture of the manor McKerrell of Hillhouse family possessed which dates back from 1671:
http://www.hillhouse.co.uk/assets/images/thumbs/Aerial-view-of-Hillhouse.jpg
Unlike Charles-Henry,his younger Bentinck brothers made "respectable" marriages to Baroness Maria Cornelia van Heeckeren van Wassenaer,Countess Helene zu Waldeck und Pyrmont and Countess Luise van Bylandt who,in the eyes of the Bentinck-Aldenburg House Laws have been good enough for them to succeed as the Head of the House and as Imperial Count...
So,here we again have a case of a mediatized family which restricted a male family member of succeeding to the title because he has made "indecent" marriage with a woman whose family of old landed gentry lairds(Scottish title below a Baron and above an Esquire) was not good enough for the Imperial Count of Bentinck-Aldenburg,but whose family was good enough for the Duke of Portland