There's a few other methods to authenticate the oracle.
1. For one, the oracle must not know what is happening throughout the trance. The oracle must have complete lost of consciousness during the whole time in trance.
2. The oracle will consume a substantial amount of liquor and leave no breath of alcohol and signs of losing control.
3. The oracle's hat if worn on normal circumstances will most likely snap the oracle's neck.
4. The oracle in trance possesses incredible strength.
I have a few uncertainties and questions in my mind.
If we have a teacher who is far-sighted and possesses clairvoyance because of his attainments, do we need to consult an oracle? Are oracles for people who rather believe in oracles rather than their own teacher who is seen as ordinary without any powers because of the humility they display?
Where does the mind go during the whole time the protector has taken over the body as a vessel. Does anybody know?
If we have a teacher of such calibre, and even if we dont have a teacher of such calibre, we should just trust the teacher all the way. However since humans are social creatures and they tend to follow the crowd rather than stick to the 1-2 people they know, some teachers find it very difficult to make the student see the truth and reasoning behind his or her ideas, therefore oracles will have to be employed.
Logically, we do not need to consult an oracle but there are 2 factors that come in mind: 1) Do we need to burden our teachers with questions that requires clairvoyance to verify? if we do but the teacher chooses to be humble or that he will see that it is not easy for us to accept his advice, he will "consult an oracle" to help convince us. 2) With so much distractions and so many teachers, if we feel that we need to listen to only our teacher and he introduces a challenging idea for us, can we accept it or do we need "backup?" If we do need backup, the teacher will again consult the oracle for our sakes.
Oracles are there to vouch for the teacher, or if the student has a question that is not proper to ask to a teacher, like for example if they have obstacles, or if they need help, how can you ask your Guru to clear your obstacles? How rude and inconsiderate!! Therefore oracles/Dharma protectors manifests so that we can direct our problems there and get the appropriate solutions for our Dharma obstacles.
For your last question, when a person takes trance, the consciousness is temporarily disengaged from the body including the brain so they are no longer able to perceive anything through their senses. They are not "outside" of the body, but still remains inside, just disengaged. The protector wil take control of the body and its winds and channels and due to a difference in energy the protector will oracle's organs will expand to fit the energy.
I am not too sure if this is how it works but through my logic and reasoning and understanding of the mind, this is the most logical explanation that i can think of.