I like most of the cast, but the actresses who play Isabella and Joanna is way too dark in appearance. They are said to resemble Catherine, so we need redheads for them as well. Say what you will about the Spanish Princess, but they at least got Catherine's appearance right. Maybe you can swap the actresses who play Germaine and Joanna? I am unsure whether or not there are any descriptions of Germaine's appearances, so as far as we know she could have been darker in appearance.
As for Isabella, I have an idea that might be a bit out there, but interesting IMO: Perhaps she could be played by Susan Sarandon? She is a redhead, speaks Spanish and although fairly old she looks much younger than her age, and she could easily play a 50s Isabella of Castille? She also looks similar enough to Catherine and is very talented, so she could shine in the role of the indomitable Isabella
Just a thought