The suggestion about an earlier birth is interesting.
Just speculating. Of the OTL Edward, in an earlier discussion someone convincingly suggested that his early life would have made him insecure, and thus probably violently defensive. Maybe most medieval boys talked a lot about killing people, but I gather he concentrated on it enough - 'nothing but' - to be remarked on.
Killing people, if done to the right people, and in the right context, is an asset to a Medieval king. Just compare Henry VI, and Edward IV in their attitude towards killing. Or even Henry VII, who killed a lot of Yorkists.
He would be no more insecure as a boy as Henry Tudor. Would he be a successful king? Maybe, maybe not.
But I'm sure he would be a lot better than Henry VI as king.
Isabella of Castile or Catherine of Navarre is a possibility.
If married to Isabella of Castille, would that lead to a union of the crowns?
No, because Juana succeeds instead.
And how would that come about?
Because Isabella is sent outside of Castile.
Isabella of Castile or Catherine of Navarre is a possibility.