I don’t see how Red John could be Kristina. Red John was involved with the secretary that killed the other CBI unit and the people that have been involved with Red John throughout both seasons refer to "him". The forensics guy is an interesting theory because that guy fits all the info that we have. Since we see him from behind when he killed the secretary with the poison in a previous episode and again in this episode, we know Red John is slight of build (which, by the way, precludes him from being the roll tide waiter). If that was his real voice then that also fits with the forensics guy.