Together with the christian socialists there were other two parties against the unification - the communists and the legitimists; both being rather insignifican party at the time.
Yes the communists were against the anschluss in 1938 but because of hitler and the NSDAP not because of a fundamental believe that austria should be an independent state.
Well, they would always have been accepted as Austrians. They just wouldn't have been accepted as Germans. German was a nationality, Austrian was a political fact. And I don't see why considering yourself a German would neccessarily mean supporting the Anschluss.
On the first point I agree on a political level they were austrians. But the difference is still relevent because it opens the door for racism. (We are talking about the early 20th century after all)
Also ofcourse feeling german doesn't nessecerly mean supporting the anschluss. It still depend on ones ideology and the contitions but the fact is that Karl Renner (social democrat), promoted the Anschluss even in 38. The argument was that Hitler would unite the german nation for now, they could worry about getting rid of him later. The Anschluss in general was still a goal for the social democrats, the national socialists and the Greater German Peoples Party. It was explicitly forbiden in the treaties of st. germain and versail and than agin by the League of nations as a condition for a loan. Only the governing christian socials (later Vaterländischefront) started to fundamentally reject the anschluss. And they lost a lot of popularity due to austartity measures and war aginst the poor in there last years. Things were already quite bad for the population so the binery choice that they had in front of them in the referendum might as well have been: "can things get any worse?"
Of course the Referandum was held under a lot of pressure so there is definitly room to argue that less than 99% truly wanted the anschluss under the conditions of 38. But if we reject the result of the referendum compleatly than we can't come to any conclusion on how many people supported the anschluss in 38.