Improving work conditions of student assistants, PhDs, Postdocs etc
As mentioned in my comment, I am in full agreement that working conditions for young scientists tend to be somewhere between precarious to downright terrible, but I do not share your impression that this is an Austrian, or even a European, problem. Still, I agree that the situation is dire and needs improvement, independently of whether it is the same or worse than elsewhere.

I have by now moved to Sweden and my impression is that working conditions for young scientists here are quite good, much better than in most countries. What seems to help are the following:


Flat management structures and a fairly consequent implementation of the "many eyes" principle. Unlike in Austria, where people tend to be at the mercy of a single chair or full professor, a young scientist in Sweden tends to work with many different faculty members, both in research and teaching. PhD committees have multiple internal members, all of which meet regularly with the student, providing realistic ways to escalate issues with the main supervisor.
Part of this are also yearly one-on-one meetings of all students with the head of the PhD school as well as with the head of the division, where problems can be raised (really, it is expected that issues are raised here, and this is not seen as an attack against the main supervisor, as it may be in Austria).
Fairly generous paid vacations on all levels, and an expectation that this vacation is indeed taken yearly.
Clear rules for hiring. There are no vague promises of jobs - all positions need to be announced and run through a hiring process in which HR is involved. This is quite transparent, and internal hires are rare to start with.
Explicit support for mental health. Our university has a contract with a mental healthcare provider, which all employees can consult if they feel stressed or depressed. Usage of this provider is anonymous and encouraged by senior faculty.


It helps that the faculty/student ratio here is much lower than what I have seen in Austria (about 1:2 in my division, versus about 1:20 in the group where I did my PhD), and that the Scandinavian culture tends to de-emphasize hierarchy to begin with.


  what can we do as young researchers/academics against an oppressive system when we are not even being represented in works' councils? Who to turn to for help and support? Are there pan-European research associations that we can become involved in to take up these issues?


Realistically, what will be needed to substantially improve working conditions will be to break up the chair structure. As long as a single professor is the sole authority over a student or postdoc, mistreatment will happen and continue to happen.

I am afraid, neither external associations nor students can do much to make this change happen - it will need to come from within. Short-term, the only thing that students and employees can and should do is vote with their feet - don't go to universities and groups that have a history of treating their employees badly, and warn others from going there if you feel you have been mistreated.