Q: Does Amstadt only apply to written representations?

I don't see anything in Amstadt that would limit its application to cases in which the alleged misrepresentation was made in writing.

Ask yourself the question of whether a DTPA consumer must (generally) prove that the bad actor made the misrepresentation in writing in order to have a good DTPA claim?

Seems to me that Amstadt is a case about WHETHER the misrepresentation reached the consumer, rather than about the format of the misrepresentation.