No, if they make a concurrent fork then they could distribute the block to many nodes in the network after they have confirmed that they node they want to attack has received the transaction they wish to reverse.
I don't understand. Can you explain in detail. If most honest nodes in the network (which run 24/7) do not overwrite any blocks which are older than 6 confirmations I don't see how they like to reverse older transactions? In the case that the merchant has only a few connection nodes and all of them are evil, he can double check on blockchain.info...
*Update
Ok here is how I understand it. You say the attacker has his blockchain 0 -> F1 -> F2 ->F3 -> F4 ...-> F10 already mined and releases F1 the same second an honest miner releases his block 1 (block 1 following block 0). The attacker than releases his block F2 exactly when an another honest miner releases Block 2 (2 following 1) and so on. Now 3 cases:
* If most honest miners now ignore F1 -> F2 -> ... -> F6 we don't have a problem because of the new security measure (as most miners and full nodes will just ignore the F-chain even if its longer)
* If most honest miners will work on the F-Chain we won't have a problem as well.
* If both chains are around equally popular full nodes will be fully aware that there is a fork and can send security warnings. Merchants who run a simple node and have only limited connections can double check on sites like blockchain.info (which should alert a big red security warning while there is a fork)