True Source = Saidin and Saidar, NOT the True Power. That distinction has existed since before we knew the True Power existed. I think "True Source" was mentioned in The Great Hunt by one of the Aes Sedai.
It is possible the 'buffer' exists only when a person is being reasonable with their ability to channel. Egwene had been on the ragged edge of exhaustion for days (if not weeks or months), and had gone insane with grief from the death of her Warder (and I can't imagine seeing so many friends die...