I always thought of the buffer as an indication of the burn-out point. Is it ever said anywhere that a buffer actually prevents one from drawing too much of the OP if they want to?

As to the Rand and Tuon conversations - I bought them. To her, Tuon represents the source of her imprisonment and torture as damane, something she clearly never got over. Of course she would lose her cool. The same goes for Rand. She believes that he is going to do something that will destroy the world. She is also determined that she is not going to be affected by him being ta'veren. It's not hard to believe that she would overcompensate, especially when part of her still thinks of him as a woolheaded sheepherder.