In spite of Freivolk's really good suggestion, the major obstacle: Since the fall of the Nazis, Austrians have abandoned their long-standing identification as Germans. The desire for a union with Germany was certainly popular in Austria in the 20s in 30s also outside the extreme right; but that had changed completely. In spite of the considerable support of the "Anschluss" in Austria, any thought of a combination with Germany would have been blemished as an occupation of Austria by Germany from 45 on.
I think your are greately exaggerating the influence the "Hitler's first victim" mindset had in the late 1940s and early 1950s. Yes, the desire to escape blame for Nazi crimes AND to get off the Soviets' thumb were the factors that eventually motivated the self-invention of a separate Austrian national consciousness. But it happened gradually during the late 1940s, 1950s, and 1960s, not overnight in May 1945. If East Austria gets communist in 1949-50, there is still going to be a very sizable amount of self-identification as Germans in West Austrians at the time, which desire to seek protection with their northern brethren will quickly and easily stoke. The main motivator to stay separate, getting the Soviets off eastern Austria, has vanished, given that the Communist coup indicates the Soviets are here to stay, so there is no great incentive as reinvent themselves as the Austrian nation. At that point, shifting the blame for Nazi crimes is not that important, the Americans in 1950-51 are already in full Cold War mood and obviously going to give West Germans a big break about past misdeeds if they behave like good pets. West Austrians may fit in nicely, no need of a separate national identity.
Moreover, the Austrian population had a strong desire for the position
the country could actually take for the whole Cold War time: A neutral one.
Communist takeover of East Austria makes the vast majority of West Austrians realize that this is now a pipedream, and makes them sufficiently angered at and fearful of Stalin that they let it go. Just like the Berlin Blockade did for West Germany.
At the same time, the remilitarization of West Germany was already planned,
or at least desired, by the US.
Indeed. And Washington shall be even happier if they can merge the resources of West Austria in that remilitarization, too.
With Austria in, that would make the situation in "Greater West Germany"
messy, and create much stronger socialist and pacifist movements.
With Communist "rape" of East Austria, I do not see pacifism in western Austrian landers be any stronger than in the rest of West Germany. If anything, with the Socialist bastion of Vienna gone, I see them becoming CDU/CSU electoral strongholds like Bavaria.
While most Germans cautiously took their hands off politics,
the unique self-understanding of Austrians as occupation victims
would provide for much more activist potential.
Which "unique self-understanding" ? East Germany and East Austria shall be two sides of the same coin, even if they stay separate. Germans in Hamburg and in Salzburg shall share the same feelings about national division.