They will inherit York and Norfolk and Brittany. That’s a lot of power to give to, essentially, a cadet branch of the royal family of England. Is that something the council/Edward would worry over?
France will probably insist on something (and Edward might agree to it) like York/Norfolk are returned to the crown (more likely York since Norfolk's willed to Richard by the marriage contract to Anne de Mowbray) and the duke of Brittany can't also be king of England.
Researching the possibility of Isabella of Aragon/Edward V match, and I don’t see how it could happen. Isabella’s future was pretty set in regards to her marriage to Afonso of Portugal. I can’t imagine Isabella or Ferdinand changing this, considering it was a large part of the Treaty of Alcáçovas. What reason could they have to break the betrothal with Portugal and marrying her instead to Edward? Wasn’t peace between them and Portugal pretty delicate at that point? Could they risk war doing so?
From what I've read, Fernando and Isabel despised/mistrusted Joao II. And he was no great fan of theirs either. That said, Fernando and Isabel would jump at the chance to have an excuse NOT to give Joao their eldest daughter. They might do a deal with him that Isabel marries Edward V while Affonso gets the closer in age Juana or Maria. Not saying he'd go for it - but he's still getting a Castilian infanta (and his son might live - if you buy the story that he was murdered by Spanish agents) for Affonso.