Next June I'll be going to Austria and I have booked a one way ticket, cause I have an italian citizenship and I'm planning on going to Italy to issue my passport afterwards. If the passport thing goes right then I'll be able to stay in Europe legally as long as I want, which is the original plan since I'd like to stay in Austria. If not then I'll come back to Argentina before the 90 days period. But I'm afraid maybe they don't let me into Austria without a return ticket. Do you think customs will be asking questions, and if they do, do you think they will understand my situation or should I get a return ticket just in case? And do you think I'll be able to issue my passport in Italy?
You will not be 'sent back', but the chances are that the airline won't ever let you board to begin with, and your ticket is lost (=no money back).
You will need to show the airline proof of your citizenship, in a way that is defined in TIMATIC (the database airlines use for that).
The easiest way is surely to apply for your Italian passport right away at the next embassy, and then travel with it.