Java test automation - Tester ask developers - What kind of test cases for testing chart Diagram

I am not really familiar with backend testing any answer is helpfull. I need to test a report functionality. The report contains statistic about transactions for the current unique user-company and it can be filtered per each company's employee id. Each company(current logged user) can have multiple employees registered).

The report is represented as two diagrams. The data objects to the diagrams are collected from a database via API=request but unfortunately whenever the testing object is being put to the database it is available to the API-request after 30 minutes (I cannot change this scheduler ).

I am not sure what kind of test cases I can create to test this from testing API-perspective and on a higher level than the unit-tests level.

My first though is to mock the testing object for the diagrams and somehow pass them to the correct endpoint, but not sure how to do it in Java

Another question is to about security, what kind of a testing is valuable here ? One company is not allowed to somehow get other's company report information.

Is database queries testing needed here?

And the last one i work in agile environment, In your opinion what are the main questions for developers who wrote this that I can ask for potentially discover as many crucial bugs as possible in a short time.

Tests are written in Java 8, Junit4. Backend is written in php 7.