[Spoiler] Who was REALLY talking while hypnotized?

Was it really Prot?

In that case has he been visiting Robert all his life so as to know the details of his childhood? I thought Prot had implied that he had only been on Earth for the past 5 years ("I have never been on such an early stage evolution planet before" or something to that effect) and that there were strict travelling regulations (hence he could not have travelled whenever his friend was in trouble)... Also, why would Prot be so emotionally affected by all this?
However if we believe the alien in foreign body theory then it must be Prot talking because Robert would be catatonic (and/or absent) and I don't think catatonic people can be woken up by hypnosis (correct me if I'm wrong).

Was it Robert talking?

In that case we can say Robert is suffering from split personality and talks as if he were Prot. He is not yet catatonic but becomes catatonic later on. But is it possible to suddenly become catatonic on a fixed date, at a fixed time?

Food for thought... I would be really interested in your opinions.