Possible to create a double spend attack by replacing a Tx in the merkle tree?
You should also know that the task of finding an SHA256 collision (same hash for two different byte arrays) is a pretty complex one. Miners are spending terahashes per second to find a hash which is smaller than a target. An exact match would be very hard to find.

Also, the attacker in this task is required to be a miner with enough power to solve and broadcast a [special] block to be included in the main chain.

In short, I can say that this task requires so much hashing power - so if someone has enough power for that, he may perform a less complex 51% attack rather than this one.