And Mary might need a different personality to marry a Protestant (since IDK how you'd get it right for her to marry a Protestant prince with her OTL personality). If he's born after Mary Grey, he's likely to be dismissed the same way as she did Charles IX of France (as too young).
You are going to struggle to have an Edward, a rough wooing thats successful, and a son to Francis Grey in the same timeline. The rough wooing happened to get Mary to marry Edward and finished because the English ran out of money, Mary was now betrothed to Francis, and in France.So in OTL, Mary and Elizabeth followed Edward. They were both older than him. Here, Edward would be followed by a youthful King, the son of Frances. Let's call him Charles after his grandfather. So King Charles Grey. He's a blank slate. He could pursue some sort of absolutist policies or be dominated by the English magnate lords. However, in this scenario, the Spanish Armada likely wouldn't attempt an invasion of England. Assuming the Rough Wooing Goes through, we also have the possibility of a United Britain under House Grey
You are going to struggle to have an Edward, a rough wooing thats successful, and a son to Francis Grey in the same timeline. The rough wooing happened to get Mary to marry Edward and finished because the English ran out of money, Mary was now betrothed to Francis, and in France.
He could be Mary's second husband, if you can get past the religious problem, and her awful taste in husbands.
Nor does the Spanish Armada completely butterfly away, Mary was from Margaret Tudors line. Mary willed her claim on the English throne to Philip. He could still use this as an excuse to invade.
Mary Stuart could leave the French court and marry Philip himself.