Not so. They all become princesses by marriage. Most are known by other titles; the only one known by her husband's name is Princess Michael of Kent as Prince Michael has no other title. For two years after their marriage the Duchess of Gloucester was known as Princess Richard, till her husband acceded to the dukedom.
///The 36-year-old former actress will not be known as Princess Meghan because of rules governing the monarchy which say only those with royal blood can use the title of Prince or Princess followed by their first name///
Exactly, they take their husbands' names. In the absence of other titles Camilla would be Princess Charles, Fergie was Princess Andrew till her divorce, Kate would be Princess William, the Countess of Wessex would be Princess Edward, as would the Duchess of Kent. Prince Philip only became a prince of the UK in 1957, ten years after his marriage, when he was created as such by the Queen. He had surrendered his foreign titles when naturalised as a citizen of the UK.
No - you cannot be a Princess unless you are born of royal blood. She will be Princess Henry of Wales, but not a princess in her own right, like being Mrs John Brown. Diana wasn't either despite being called that, she was the Princess of Wales by being married to the Prince of Wales. So Meghan will only be a Duchess.