He wasn't wrong, though. After feeling the vastness of the DO, he gave up on killing him in his prison as he had told Moiraine he was going to do. He never explicitly stated the DO was unkillable.
He was not wrong at the start. He dropped hope of killing the DO after entering his prison - he never said anything about it not being possible.
You are missing our crucial point. They had knowledge of everything, direct, verifiable knowledge, of everything sans the DO. The Forsaken, AoL...
It says they sensed a source of energy. This is most probably the TP, though it may not be - the negative energy of space is considered tremendous, and as RJ was a physicist this may be the answer.
Being an expert on Pattern metaphysics is far removed from knowing anything about the Dark One...
False. The Bore was drilled because the TP was detected, and if it could he felt, it could probably affect the Pattern. Remember, we are told he is not allowed to touch it while Sealed away. This is different from having some sort of entropic effect from a distance.
Saying that a Third Age...
Seanchan people disgust me. I'm glad Mat is going there, because he has a wierd way of making jerks and smug idiots look like fools. If anyone can make Seanchan a land of not-absolute jerkoffs, Mat can.
Rand appears to be way stronger in the Power than Lews Therin. He had saidar, which when combined with saidin can create a LOT more powerful effects than saidin or saidar on their own. He also had the TP, and didn't have to divert some of saidin to holding back the DO.
As for the "impossible pipe"... he's clearly not holding onto any sort of Power, so that rules the OP, saidin or saidar out. The Bore has been closed, so I don't think it's something with Tel'aran'rhoid, as that was mentioned as moving closer to the real world as the DO's influenced increased...
I think it does say that the One Power became one... that saidin and saidar were no longer divided. But what exactly did he DO with it to seal the Bore? It's been bugging me.