The French, IIRC, were never too keen on Anne (as Queen) or Elizabeth (as heiress); it took a whole lot of pressure from Anne on Henry to eventually convince the French to stop asking for Mary's hand (they still saw her as the heiress) and agree to Elizabeth being very tentatively betrothed to, I believe, the Duc d'Orléans. I'm not sure the French would be too supportive, unless they truly believed Mary would embark on a Spanish marriage (which we all know, but which they themselves obviously didn't consider as the only possibility).
Whilst Wiltshire and Norfolk would be best positioned to take power immediately, I don't think either would remain in the long run. I also see the Reformation being rolled back almost immediately. Henry and Anne only went in the direction of the Protestant German princes because they were being shunned by Catholic Europe, but with Henry dead the situation's at least partially resolved. Mary was popular, as was FitzRoy; Henry VIII would've had the power to will the throne to FitzRoy, no problem, and/or Norfolk would had the power to set him up as King in opposition to the child.
Add Courtenay and Charles Brandon into the mix and you've got another War of the Roses.
No, but sometimes war makes for strange bed mates. Charles was always Katherine of Aragon's partisan and always Mary's, as well. Indeed, she always considered him her chief adviser, sometimes to the detriment of England. They'd almost immediately move to aid her if Henry VIII died, if Spain made a move. Remember, this is the time of the Italian Wars. England isn't a great power, she's just a chess piece in the Valois-Habsburg games.
As for Fitzroy, he's also an important factor. Henry
could've named him as his heir, but he didn't. He never planned too either. He had a very keen idea that he wanted a
legitimate son, hence his divorce. His playing with Protestantism was thus for that, but Anne herself was keenly Protestant. She had and was even allowed to read books that were on the forbidden index. She most definitely had sympathies for the movement as did her father. She's not going to roll it back, especially when her opposition is Mary. Henry's Protestantism was skin deep, for sure. But Anne's? I find that
very hard to believe.
I just think in a situation like this, Fitzroy doesn't stand a chance. If everyone wears themselves out, he'd be able to play up English patriotism and perhaps place himself as a third way between Mary and Anne Boleyn, but no one is going to actively champion for him. Norfolk was pretty attached to the Boleyn camp, IIRC. He's not going to resist possibly being Regent for a baby girl when the other choice is Katherine of Aragon's daughter and Henry's bastard, both old enough to rule.
Fitzroy could play Monmouth, but he's just like him too. Had he lived, Henry was never going to name him his heir. He believed his marriage to Katherine cursed and wanted a new marriage. He was never Protestant, of course, and only broke with Rome formally. But Anne at least had reformist leanings. I just don't see Anne rolling back the reforms. Why would she restore a religion that would essentially proclaim all she had been was Henry's mistress and Elizabeth his bastard? That's not in her character. She was too ambitious for that. Anne, like Mary was very concerned with the meaning and would never ever accept the bastardization of her daughter while she lived, while Mary could never accept that she was illegitimate. Mary's rolled back Henry and Edward's reforms and decreed the divorce void, retroactively making her legitimate again. Elizabeth never challenged any dispute regarding her parents marriage, or attempted to assert her legitimacy; she knew that
her religion, the
English religion was more than enough.
But a surviving Brandon and Courtenay add to the mix, as does a surviving Margaret Tudor and her offspring. If Henry dies shortly after Elizabeth is born, it also leaves England without a will. Hence the succession is open. It follows male preference primogeniture of course, but it was Henry VIII who opened up the can of worms by decreeing the line of succession and excluding Margaret and her heirs. Here it is clarified, at least a little: Mary or Elizabeth come first, depending on the view of the divorce and marriage in eyes of the Catholics & Reformers; then comes Margaret Tudor and the House of Stewart descended from her. Mary and her offspring. Then from there the House of Tudor is extinct, and it'd follow the remaining extant Plantagenet lines. It does leave an interesting position, as Parliament will have never gained the right to "settle" the succession perse. Henry's marriage with Katherine was annulled of course, and Mary declared a bastard, but nothing like the changes Henry instructed in his Act of Succession.