I don't think Banderas as Ibn Fadlan was a miscast. Banderas is from Malaga, Spain, a city with 5 centuries of Muslim history (at the time of the film's plot it was part of the Umayyad Emirate of Cordoba) and his family probably has Arab roots, like all of southern Spain. Therefore, it is not so unthinkable that Banderas could interpret an Arab from the Abbasid Caliphate in 922. Ethnically, Banderas can fit the character.
When she said that the character should be played by a Muslim actor, it is always good to remember that Muslim is not a race, it is a religion. There are Muslims of all races, not just Arabs.
Fact: Omar Shariff was born a Christian, in a Lebanese Catholic family, and later converted to Islam.
By the way, the real Ibn Fadlan was an ambassador of the caliphate, sent to the khanate of volga bulgaria. He wrote a chronicle of his journey, Rislan, where he describes the different cultures he has encountered along the way, including the Vikings of Russia.