I'm confused on why you are using reflection. Everything above does not need it. All are public except for the method which just returns itself so it doesn't matter. They are all forge registry entries which you can register normally.
To begin where are you trying to access info for the container? From the server or client? If it's the latter you'll need to send a C2S packet. Then the easiest way to collect info is when the player opens the inventory, since they have a containerMenu variable. The packet would look something like: