To elaborate, a merchant can easily monitor (or pay someone to monitor) transactions being sent to miners in real time. The probability of a successful double spend decreases drastically as the time since the first spend increases. If someone hasn't tried to double spend in ten seconds or so, they'll likely fail if they try.
This is simply false. Many mining pools allow you to push to transaction to them. If your first spend has a 0.0001btc fee and the second has a 0.001btc fee, they will choose the 0.0099btc fee.
You cannot monitor transactions being sent to miners if they aren't propagated through the normal bitcoin network.
Along with that, there have been cases where mining pools have doublespent unconfirmed transactions and stolen lots of bitcoins. GHash stole 3000btc from betcoin https://bitcointalk.org/index.php?topic=327767.0
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