"Est-ce que tu as tout rendu?"
"Did you return it all?"
Should "it all" or "all of it" be considered equivalent to "everything" for this sentence structure?
Yes. They are now all accepted.
''did you send everything back'' not right?
why not: did you return all?
Why not did you all return?
First, you cannot use "tu" in the plural form, so "you all" is automatically wrong. Secondly, "tout" is the object of the verb "rendre", so the sentence means "did you return everything" or "did you return all of it".
When I looked up rendu, it came up as a form of rendre (make; visit; report).
How does that translate to "return"?
Did you look at all the definitions of "rendre"? There are a lot of them. See here: