Diana was never Princess Diana. Only a Princess born as a Princess in the UK can use Princess and their own names e.g. Princess Anne, Princess Charlotte etc.
Wives of British princes use their husband's names so Diana was Princess Charles.
Fortunately for her Charles had higher titles than simply Prince for her to use 'Prince of Wales', 'Earl of Chester','Duke of Cornwall', 'Duke of Rothesay' etc. Diana, like Camilla, could use the feminine form of all of these titles - so Princess of Wales, Countess of Chester, Duchess of Cornwall, Duchess of Rothesay etc. The only one of these titles Camilla has not used is Princess of Wales but she is entitled to do so. As she uses Countess of Chester she has also to be Princess of Wales as both Prince of Wales and Earl of Chester were given to Charles in the same Letters Patent.
Diana was stripped of HRH at the time of her divorce,which is also when Sarah lost HRH. Sarah had remained HRH from the time of her own divorced until Diana's divorce.
After their divorces they followed the custom in the UK and used their own names with the feminine form of their husband's senior title so HRH The Princess of Wales became Diana, Princess of Wales and HRH The Duchess of York became Sarah, Duchess of York.
I think Diana could have reverted back to her own courtesy title and could have used Lady Diana Mountbatten-Windsor as well.