I think you could easily support LIMIT+OFFSET and empty sort key, e.g. select * from "users" limit 3 offset 2, pushing "limit 5" down to cassandra if cassandra doesn't support offset. It's worth adding that test case either way.
Similarly, I don't know whether cassandra supports sort+LIMIT (i.e. sort with LIMIT and non-empty sort key) but you should add the test case whether or not you push sort+limit down, just to make sure that nothing bad happens.