Not really. There are always lots of 'versions' of the blockchain floating around. The network only keeps track of the longest chain (broadly speaking). This means the network with 51% can determine which transactions get into the blockchain. For example, if the pool owner doesn't like you, he can essentially 'blacklist' your account, which means your Bitcoins can become unspendable, basically. The longer they have the 51% power, the more damage they can do. GHash already performed what you describe as your '49% attack' against a gambling site that accepted 0 confirmation deposits. There has been no known instance of a 51% attack yet (e.g. double spend after 6 confirmations).
You pretty. Much have to detect the fraud before/as it occurs - the dominant miner being silent for a while is a good indication they are building an alternate chain in private, for example. At least, that would be an indicator for a double-spend.
Not really. That happens regularly just because of luck.
You'd see that every single block was mined by the same pool.
That doesn't mean a 51% attack happened, it just means someone got 51% of the network hash rate.
An honest miner with 51% of the hash rate would only mine 51% of the blocks (example: https://blockchain.info/blocks ); an attacker would mine every single block.
How detectable would such an action be? Wouldn't other systems be able look at the block and say "it's verified, but it don't look right to me"