How can I (re-)create the Entity Relationship Diagram (ERD) of an existing hidden Oracle database?
Many database modelling tools can extract the required data and create an initial ERD diagram. You will likely need to rearrange the diagram, but the initial diagram will provide a good starting point. 

Given the limitations, you may need to use queries that generate DDL and create a database based on this DDL.  I would avoid selecting from the all_* views as many of those objects will be system related, rather than application related.