An allusion is an expression designed to touch a subject without doing it explicitly, we could say its an 'indirect' way to say something (most likely controversial).
In this letter Martin Luther King uses this technique to reference civil rights activists and clergymen which gained him mass support since they could relate to his point of view. Throughout the letter he justifies his cause in the name of justice.
Depends on how major or minor the errors are. If the errors are due to lack of knowledge then it isn't a sign of progress but if less errors are being made and they are small ones progress may be shown.
