mysql_prepare_create_table() does data validation and such utilities as automatic name generation. But it does that only for indexes and ignores Foreign_key objects. Now as Foreign_key data needs to be stored in FRM files as well this processing must be done for them like for any other Key objects.
Replace Key::FOREIGN_KEY type with Key::foreign flag of type Key::MULTIPLE and Key::generated set to true. Construct one object with Key::foreign == true instead of two objects of type Key::FOREIGN_KEY and Key::MULTIPLE.