Western scholars: Did the Buddha not speak the Digha Nikaya?
Is there evidence (such as contradictions with other suttas)
suggesting the Buddha did not speak the Digha Nikaya?


The four primary Theravadin Nikayas, namely Digha Nikaya, Majjhima Nikaya, Samyutta Nikaya, and Anguttara Nikaya contained no contradiction with each other.

You may also wish to scrutinize the Buddhist Councils "convention" and their outcomes to ascertain the level of "authenticity"/preservation of the Buddha's teachings.

In there any evidence in the suttas where the Buddha said he would 
teach a different modified Dhamma ("propaganda") merely for the 
purpose of converting Brahmins &amp; other outsiders?


In the suttas, the Buddha pretty much taught the same thing to everyone. There was no "special" teachings for select groups of disciples or audiences.

Is there any evidence in the suttas showing the Buddha was "flexible"
in his teaching of Dhamma? Or do the suttas show the Buddha wished 
his teachings to remain consistent &amp; clearly representative of what 
he taught?


The Buddha had over 40 years to teach. So, he did teach the same thing in many ways depending on the intellectual level of his listeners. And he had 40 years to admonish/correct disciples who developed wrong views of his teachings.