What about the concept of detecting double spends in bitcoin nodes and notifying merchant? ... If no second transaction is seen in 10 seconds it is considered 'safe'.
It would help further if nodes that detected double-spends either relayed them, or, better, relayed an alert that a double-spend was being attempted.
Good questions. Everyone sees that the decentralized bitcoin system can't have complete, immediate protection from the effects of 0-conf double-spends, but that is totally different from saying that the situation cannot be improved immensely. Some improvements that can be made are:
1 - Agree that 0-conf double spends should be made more difficult, not easier (there are actually people who disagree).
2 - Per inner quote above, wallets should immediately and loudly notify their users of double-spends. Amazingly, I am not aware of a single wallet that does this! Please post if I am missing one or more. The reason you notify the user is that they may not yet have handed over the merchandise.
3 - Per outer quote above, the network should relay, and not drop, double-spends. This makes #2 above much more effective. The actual double-spend transaction is the best alert, because it is signed by the person double-spending. Anything else would require a trusted 3rd party.
5 - Longer-term, before giving up on the bitcoin protocol, we should ask how to incentivize miners NOT to include second-spends in blocks. For example, I have never seen a reason why this idea from 2011 could not reduce the unsafe period for 0-conf double spends to only seconds, instead of many minutes
https://bitcointalk.org/index.php?topic=3441.msg48484#msg48484