I think I just had a realization and I wanna make sure it is right...
Is the prefix "till" in Swedish the same as "zu" in German? For example, tillsammans is zusammen, tillbaka is zurück, etc? As a general rule, maybe not always (like the prefix zu in German is like toe in Dutch, but not always, because zusammen and samen)...?
They are equivalent, yes. They aren't related however, despite the similar appearance.
tillsammans = zusammen
tillhöra = zuhören
tillstånd = Zustand
There are tons of such words due to historical German influence on Swedish.
Yes, in general: till = zu = to. Of course prepositions always cause problems because they are not used consistently across languages.
Vi alla arbetar tillsammans = we all work together, but that's still different than "we work all together" which would imply maybe a lot of people doing an action together all at the same moment.
This sentence is just saying that two(+) people work together. If you're my colleague, då arbetar vi tillsammans :)
Not really, at least to me that sound as if the work is done on the other person, like two psychologists discussing their issues together.
Here I'd say they are synonomous. But sometimes "ihop" means making/having many things into one, but "tillsammans" it is still multiple things. So you can "sätta ihop ikea-möbler" (put together ikea furniture) but not "sätta tillsammans ..."