Hello Immanuel,
I looked at all of your posts and thought I should comment on a couple of your issues. It is not so easy to understand some of what you write as it sounds like you are not a native English speaker or are very bad in expressing your thoughts. Please don’t take this as an insult, it is NOT meant as such but one needs to be open about any communication problems.
VHS is usually the cheapest place to find German courses and even on a beginning level they are given in German rather than English. There are a couple of reasons for this, probably the main one being that foreigners from all over go there to learn German and may not know much or any English and one cannot expect the teachers to explain things in everyone’s native language.
You write about an injury and asked why they would ask if you still have pains. It is of course a more serious thing if one claims to now be unable to work for an indefinite period of time or to be permanently disabled. If one is just hurt for a couple of days it will probably be seen more like getting sick leave. But it is important to make sure any doctor writes that an injury happened on the job.
As you have experienced, part time jobs lack most benefits. There are only appropriate for spouses who have a full time worker and the other wants to make a little money. Especially mothers who don’t have time to work full-time anyway, might take these jobs. The only solution to get benefits is to find a permanent full time job but till you do, your health insurance should at least get covered by your spouse’s but you will not get any credit towards retirement.
There are laws about working conditions in Germany but it is an illusion to think that they are well enforced. Many jobs, especially at the cheap supermarkets like Aldi, Lidl, Netto etc. are well known for misusing their workers, often expecting people to work overtime without pay, expecting more work than one can manage and to fire people after years of good work just so that they can take cheaper beginners. One can complain and make lawsuits but this is VERY difficult. One has to pay lawyers and fight big companies that want to protect their business model of keeping labor costs down to a minimum. One has the burden of proof and the fact that the German court system is extremely slow, often taking years for even a simple case. I had a company cheat me then declare bankruptcy. The court appointed someone to oversee the case but they told me to expect it to take at least 5 years to be resolved and that one usually gets a few cents per Euro owed. In other words, one has little chance of getting anything.
There was just a TV report some days ago about Netto supermarket and how they mistreated employees for years. These people have lots of documentation yet will probably never get much anything. The upshot is that one should look for a good, fulltime job and unfortunately accept that most part time ones are crappy and there is not much one can do about it. Many foreigners come to Germany and expect efficiency and that the trains run on time and that things like this don’t happen. But there is actually a lot of corruption in many areas; it’s just a bit more hidden. I don't mean to discourage you from finding out your rights but just don't get too high expectations that anybody will do much about it.