In his Introduction to the Fiftieth Anniversary Edition of TRFTR, Ron Rosenbaum commends Shirer for "bring[ing] forth from the shadows of amnesia this nearly forgotten figure, an acolyte of the Pan-German League, who may have been the most decisive in shaping—distorting—the pliant young mind of Adolf Hitler" (A New Introduction).
As Shirer notes, Leopold Poetsch was one of Hitler's childhood teachers—the only one, in fact, whom the young would-be Fuehrer seems to have liked. In Mein Kampf, Hitler writes of Poetsch, "He used our budding national fanaticism as a means of educating us, frequently appealing to our sense of national honor. He made history my favorite subject" (1.1.59-60).
Somehow, we're not feeling the same gratitude.
Given the warped sense of history that Hitler carried around with him for the rest of his life—and which he continued to build on through his own self-directed "studies" in human development—Poetsch won't be getting any retroactive teaching awards anytime soon.
As the president of Austria at the time of Nazi Anschluss, Miklas—like Chancellor Kurt von Schuschnigg—took part in signing away Austria's independence. But whereas Shirer tends to describe Schuschnigg as a somewhat naïve and unperceptive man, our author seems to have admired Miklas's stubbornness and obstinacy.
Unlike Chancellor Schuschnigg, Miklas refused to resign his position as President when the Nazis threatened him with force. Instead, he was determined to wait until the Nazis kicked him out, which they quickly did.
Kurt von Schuschnigg enters The Rise and Fall of the Third Reich as the Austrian Minister for Justice, but he reappears again later as the Chancellor of Austria.
In Shirer's view, Schuschnigg helped to pave the way for Austria's downfall by "embark[ing] on a course of appeasement" with Hitler (3.9.84). As he reviews the Austro-German agreement that Schuschnigg signed in 1936, Shirer argues that the concessions it included pretty much gave Austria away.
After the German invasion of Austria, Schuschnigg was treated abominably by his Nazi captors (surprise surprise), who subjected him to degrading treatment. He was eventually sent to a concentration camp, and was fortunate enough to survive until the Allied victory in 1945.
Seyss-Inquart was a Viennese lawyer who not only sympathized with the Nazi Party, but was determined to help them get control of Austria so that Hitler's vision of uniting Austria and Germany could be realized. Through the concessions that Hitler squeezed out of Schuschnigg, Seyss-Inquart acquired ministerial power in Austria so that he'd be in a prime position to advance a Nazi coup.
As he describes Seyss-Inquart's role in the Nazi annexation of Austria, he repeatedly emphasizes his treachery, describing him as "the first of the quislings" (3.11.64), Austria's "Number One quisling" (3.11.99). He became the Nazi Governor of Austria following the Anschluss, but when the fall of the Third Reich eventually came, he was sentenced to death at Nuremberg and executed by hanging (Epilogue.9).
Btw, "quisling" means traitor or collaborator; it came from Vidkun Quisling, a Norwegian politician during the war who collaborated with Nazi Germany. It's like Americans might use the term "Benedict Arnold" to mean a traitor.