Aur agar yaqeen ho to?

As for as Istebra ( cleaning an animal and bird by giving them Halal food) concern, it is restricted to human refuse only. In other case as you mentioned if you are sure then it’s better to avoid such meat. Therefore if broiler chicken has been fed with food which was mixed with haram food ingredient then it’s better to avoid to use it otherwise it’s just like many halal meat birds they eat insects which is haram but their meat is not considered as haram as long as they should not start feeding Human refuse.