No. The block doesn't contain the 50 BTCs. The block "allows" the miner to pay him/herself 50 BTCs if the block is below 210,000, at 210,000 that payment goes to 25, and every 210,000 blocks it halves again, until it is below the maximum resolution of the software in which case it is effectively zero. Yes. It contains the transaction fees for the transactions described in that block, which the miner who "mined" it gets to pay themselves.
Yes, the difficulty of mining CAN decrease (though I doubt it's happened) if the hashing power of the network decreases. This is to keep the predictable amount of bitcoins coming into the market. If all the miners left but one, the network still needs to create bitcoins at a predictable rate, so the difficulty would go WAY down.
By the same token, when the hashing rate explodes (as has been happening) the difficulty ratings will soar, so the same bitcoin production rates are maintained.