I think political convenience only has a part to do with it. This is England, not the Ottoman Empire. France as well as Spain & the Empire, are going to be backing their favorite horse to win.
So, as much as England would like to believe it's above continental politics, if Lord Henry Darnley can get the backing of say France and Scotland (due to his mother being the late king's uterine half-sister), whereas Huntingdon or Courtenay can only field an army supported by their estates - Darnley IMHO will be Henry IX. It'll be a case of "Until someone with a bigger army shows up, I'm in charge"
NOTE: I didn't let the Habsburgs back anyone of the 3 contestants, since Karl V is going to be backing Mary. Blood is going to count for something there, even if she marries say Courtenay, and maybe Liz can marry Darnley? Or some such arrangement will be reached.
Maybe even a Edward VII and Mary I, or Henry IX and Elizabeth I? Is a joint monarch scenario possible?