Remember, if they return Katherine of Aragon to Spain, her dowry goes with her. Which is why she ended up with Henry after Arthur, H7 wanted to keep the $$. Romantic, isn't it? And Katherine had been going to be "Queen of England" almost since birth. She'd be loathe to give that up, even if means wedding Edward.
She was very attractive at that time, do not forget that. That's how E4 ended up with his Woodville bride, she was very attractive and didn't put out - no ringee, no flingee.
Desmirelle, Katherine and her dowry stopped to be attractive some time after her mother’s death, plus the whole dowry was never paid and little remained of it at Henry VII’s death...
The only reason for which Henry married Catherine was who he was in love with her (plus she was there and ready for babymaking)... Eleanor of Austria (aka Henry’s official fiancé) is a better prospect under any point excluding age so if Edward (like his father) do not fear so much the risk of the Stewarts inheriting England he will marry Eleanor.
A Katherine sent back in Spain is one of the two biggest candidate for marrying Manuel after Maria’s death and the most logical one (the other would be the more risking Queen Dowager Germaine)
Aha. That might require Philip having a third son, to inherit the Empire.
Well, Maximilian can also have a son by Bianca Maria (who will marry Anne of Bohemia and Hungary)
But with Catherine back in Spain you can have easily a Ferdinand as King of Spain and married soon to Isabella of Portugal (with many kids and a secured succession) with Charles still alive and ruling Flanders and Austria.
I can not see Charles marrying Anne Jagellon so she will likely marry Zapolya after Maximilian’s death (and after Louis’ death I see Hungary to Zapolya and maybe to Polish Jagellons after that and Bohemia to Charles)...
A Spain/Austria split of this kind will strongly influences matches there so Juana’s kids will be married slightly different than OTL...
Charles, HRE, Archduke of Austria and Duke of Burgundy: Mary Tudor the Elder or a French royal princess most likely, Anna Jagellon more unlikely
Eleanor of Austria: Edward VI of England
Isabella of Austria: Christian II of Denmark
Mary of Austria: Louis II of Hungary
Ferdinand VI of Spain: Isabella of Portugal quickly
Catalina of Spain: João III of Portugal
Manuel I of Portugal (as third wife): Catalina of Aragon or Germaine of Foix