I doubt they bought two tiara's for a wife that was not really in the cards. There have been suggestions that some wanted him to marry Juliana of the Netherlands but that would have lost him the inheritance rights in Belgium as Belgian dynasts are by law forbidden to marry an Orange-Nassau.
I have not heard of any other proposed bride for the count of Flanders. The two youngest Luxembourg princesses were only one and two years older but both married in the early 1920's and by that time Charles was barely an adult. One of the Italian princesses might have been an option however there were speculations about his brother Leopold marrying one of them but I don't remember ever reading anything about him.