Not sure why it was done. Maybe they wanted to remove that German part because they wanted to make it clear that they are different from Germans? Why was that name created in the first place?
Stipulated by the allies and the Treaty of Saint Germain. Which forbid Austria to seek unification with Germany. Pretty much the opposite of what you suggest.
IIRC the feeling was that Austria itself wasn't really able to sustain itsself given that the breadbasket was Hungary, so joining up with the Germans was considered likely.
Not sure about the reasoning behind the regulation laid out in the treaty, but not having a single germanic bloc in the middle Europe is a good guess.