If a Post Commander does not have the authority to arrest unauthorized people from carrying firearms on post, I guarantee Congress will create the law which gives him that authority.
Just get yourself arrested, appeal it through the courts, and win your case.
Then Congress will change the law, and then next time you get arrested carrying on post, you will stay in jail.
This is testable, not like philosophy, it can be tested.
If I'm not shooting, I'm reloading.