Thank you for the link, definitely appreciated.
As you say:
The answer is, they can't. The Steem witnesses, and indeed, the existing Steem chain at large, will have nothing to do with the TRC STEEM token.
Then again, I was not aware of the Polo token swap announcement you mention: https://support.poloniex.com/hc/en-us/articles/360043406513
It may all come down to how the exchanges respond and how the liquidity in the market responds.