So heres a thought of what i beleive they were going for in Seto's crying scene

I believe it was indeed tears. When someone is petrified they are for all intents and purposes a rock. That is why, when your entire party gets petrified, its game over. You would in theory not die, but instead stay in a state of suspended animation unitl someone softed you. My theory is that in the petrified state, your cells would stop moving, stop aging, and consequently this would suspend you for an indefinate period. Seto however was apparently an extremely strong warrior. Bugenhagen (not sure on spelling) said that Seto continued to protect the valley even after he was turned to stone. This suggests that he was capable of overcoming its effects, for some period of time, though sheer force of will. those action could have caused his current state (why they couldnt just soft him). He may be able to overcome these effects temporarily at will. Remember that Bugenhagen also said that Nanakis mother and father both agreed to have the cave sealed. This suggests that he must have overcome it for atleast enough time to agree. It would seem however it takes a highly emotionaly filled situation for him to do it. Something like, Ohhh I dont know, Seeing his son for the first time in years and hearing his son gain admiration and respect for him. Anyway its not something provable but to me it makes sense.