As far as I understand it:
1. He learned how to manipulate the Pattern during his metaphysical battle with the DO. Therefore, he can now warp reality at will.
2. Basically what Lidolas said, crossing balefire streams linked their souls.
3. Nakomi is likely to be an avatar of the Creator.
4. The boys' status as ta'veren is no longer needed by the Pattern, the Last Battle is won, therefore they have no reason to continue as such and risk (as the Pattern sees it) more complications down the line.