I see no new ground here except "decentralized exchanges are good" (and "forks are bad").

I agree except that doesn't really solve the problem, not even in the future, since exchanges are just one example of a good being delivered rapidly (other coins in this case). With any other commerce transaction where the goods or services have been delivered double spending leaves someone holding the bag with no recourse. It doesn't really matter if the blockchain is traceable or not.

Also, the blacklist issue is greatly reduced because a blacklist as you propose would only be effective if put into place before mixing occurs. Once the mixing occurs, you can't undo it, and you can't effectively blacklist the root coins because far enough back you are essentially blacklisting all coins. Not that far even, given the exponential spread of mixing.

EDIT: Also, there is still no credible basis for a private key attack due to either de-anonymizing (traceability) or double spending. It hasn't been be disproven (indeed most of practical crypto is strictly speaking unproven), but continuing to repeat it as pure "there might be a flaw" is just uncertainty and doubt with no analysis backing it up and is not credible.