So if Rand and his friends Sealed the Dark One away properly, neither half would be tainted.

If anything, this might be a giveaway that the Seal was done successfully, and the Seanchan Empire is now perpetrating evils that will destroy the Westland cultures. But, honestly, since Aviendha saw a possible future, this is just one of many possible outcomes - as Min said and Cadsuane confirmed, just because Foretellings, Viewings and prophetic statements were still being delivered didn't guarantee that the future would still proceed - if the Dark One broke free, all of reality would be destroyed and time would lose meaning.