Genealogy is an excellent hobby. I'm sure you've enjoyed all you've learned along the way.
However, your specific question "William I is my 28th Great Grandfather. His 24th and 25th Great Granddaughters are Queen Elizabeth II and Princess Diana. What cousin are they to me" would require knowing the complete genealogies of all three families.
In giving a genealogy one either starts with oneself and gives parents, grandparents, etc. or you start with the oldest known ancestor and give all children, grandchildren, great-grandchildren to the present day (including ones who are not in their direct line).
I can't see in the list of names you've given how you are related to Queen Elizabeth II (daughter of George VI of England and Lady Elizabeth Bowes-Lyon). I also don't see an immediate connection between any of the people in your list and Lady Diana Spencer, daughter of John Spencer and Frances Shand-Kydd.
If your arithmetic is correct and you are the 28th great-grandson of William the Conqueror, that still doesn't give a clue as to what relationship you would be to the hundreds of thousands of others who are also his descendants. The descendants of William I have been well documented. Here is one list: http://familypedia.wikia.com/wiki/Famous_Descendants_of_William_the_Conqueror_of_England
Please provide more information and I will be glad to see try to see what cousinship you might share with the Queen and the late Diana Spencer.