The problem I see is the miners should be able to write, in the blockchain, the proof a double spend was underway at the time of they took the coins away.



I don't like the miners appropriating money for any reason. This is not their job description.



I suggest waiting for "Weak blocks" development and other improvements to allow miners to know, in advance, the contents of blocks other miners are working on.



With this approach, a 0-conf transaction would be demonstrably in the blocks the miners are working on. The merchant would know, roughly, how much of the total hash power (and miners) would include tx one and how much would include tx two.



Anyway, in presence of a double spend, the merchant should just waits.

If he wants to accept a 0-conf transaction being double spent it is his problem.