I don't feel like belabouring the first point more, you'll see a bad thing happen when it happens. It has nothing to do with SHA256d preimage resistance.
Which aspect of orphans worries you?
However, proof of stake, as implemented in nearly every currency so far, has one fundamental flaw: as one prominent Bitcoin developer put it, theres nothing at stake. The meaning of the statement becomes clear when we attempt to analyze what exactly is going on in the event of an attempted 51% attack, the situation that any kind of proof-of-work like mechanism is intended to prevent. In a 51% attack, an attacker A sends a transaction from A to B, waits for the transaction to be confirmed in block K1 (with parent K), collects a product from B, and then immediately creates another block K2 on top of K with a transaction sending the same bitcoins but this time from A to A. At that point, there are two blockchains, one from block K1 and another from block K2. If B can add blocks on top of K2 faster than the entire legitimate network can create blocks on top of K1, the K2 blockchain will win and it will be as if the payment from A to B had never happened. The point of proof of work is to make it take a certain amount of computational power to create a block, so that in order for K2 to outrace K1 B would have to have more computational power than the entire legitimate network combined.
In the case of proof of stake, it doesnt take computational power to create a work instead, it takes money. In PPCoin, every coin has a chance per second of becoming the lucky coin that has the right to create a new valid block, so the more coins you have the faster you can create new blocks in the long run. Thus, a successful 51% attack, in theory, requires not having more computing power than the legitimate network, but more money than the legitimate network. But here we see the difference between proof of work and proof of stake: in proof of work, a miner can only mine on one fork at a time, so the legitimate network will support the legitimate blockchain and not an attackers blockchain. In proof of stake, however, as soon as a fork happens miners will have money in both forks at the same time, and so miners will be able to mine on both forks. In fact, if there is even the slightest chance that the attack will succeed, miners have the incentive to mine on both. If a miner has a large number of coins, the miner will want to oppose attacks to preserve the value of their own coins; in an ecosystem with small miners, however, network security potentially falls apart in a classic public goods problem as no single miner has substantial impact on the result and so every miner will act purely selfishly.
http://blog.ethereum.org/2014/01/15/slasher-a-punitive-proof-of-stake-algorithm/