Desmirelle at that point Mary was the legitimate daughter of the King but never formally declared his heiress and at that point ENGLAND NEVER EVER IN HIS HISTORY HAD A QUEEN RULING IN HIS OWN RIGHT and all the precedents were against Mary.If Mary is engaged to Charles V at the time of Henry VIII's death; Mary will be Queen or there will be war. This is the 16th century we're talking about and this is a country we're discussing. Spain and the Empire will always want to be bigger and to acquire England, the money will be found. Every nation at this point believes Mary to be the legitimate daughter of Henry VIII and Catalina of Aragon; that's who gets the throne. This is not France with Salic Law, there's actually no reason she can't be Queen. And if there's a war and Mary wins, Charles will wed her at once, delaying consummation. Now THAT would be the real nightmare for England.
The power struggle in England will be the regency, not the crown. Norfolk's going to back Mary Tudor; Suffolk's going to back her for damn sure; Wolsey's problem with it is that her mama don't like him; Cromwell's still up and coming, and the people of England LIKE her (and her mother). The nobles may well break her betrothal to Charles AFTER she's queen as part of the regency policy. But by-passing her is a recipe for disaster and war.
Look what happened to any potential heiress of the English Crown:
1) Matilda of England, mother of Henry II: her cousin Stephen (younger son of one of her father's sister) usurped the Crown and was able to hold the Crown almost until his death. Matilda's son Henry was able to take England after a deal with Stephen who made him the heir of Stephen. Matilda was still alive but never became Queen
2) Eleanor of Brittany: imprisoned all her life by her uncle John I who had taken the crown, killed her brother and feared her claim
3) Philippa of Clarence: was said who after her father's death, the King her grandfather excluded her by the line of succession. Her cousin became king instead of her heirs and while her descendents were able to reclaim the Crown they were also male line descendants of another of her uncles.
4) Margaret Beaufort: Henry VII claimed the crown as son of Margaret but she was alive at the time and never tried to claim the Crown.
5) Elizabeth of York: Eldest daughter of King Edward IV, daughter of a crowned King and Queen was likely the most legitimate heiress of the English Crown, became Queen consort of Henry VII, who married her only after being crowned.
The Tudor Dynasty at that point had only two Kings: Henry VII and Henry VIII and Mary was never recognized as heiress by her father.
You have not any reason for being sure who Mary will become Queen or Charles will declare war for Mary.
What all the precedents said is who the most likely outcome is either James V of Scotland or Henry Brandon (if the first is excluded being the king of another country) will be declared King and then engaged to his cousin Mary.
Why you think Henry VIII who was in his heart clearly a true Catholic risked the war with Spain and breaked with the Pope? Because he wanted Anne Bolyen? You are deadly wrong. Henry wanted so desperatly a male son because everything in the English's history said who a woman was unable to become Queen and hold the throne. The Tudor's dynasty was young and unstable and was unlikely who little Mary would be able to hold her Crown. English people liked her and Katherine? Sure, but that does not mean they will fight for her against a more substantial candidate. Mary is legitimate but a woman and if the council decide to formalizing who a woman can give the rights to the crown to her male descendants but can not become Queen? Everything point in that direction