The only thing that stops a miner from crafting a chain in private is the cost associated with running the mining equipment and not claiming the block rewards.  If they have >50% hash power, then they have a near 100% chance of having the longer chain and making back the block rewards as well as their double spend.  It is very possible that a government or some large entity could mine a longer chain in private and release it to disrupt the network.  I'm not sure why you think this is not possible with PoW.  The other stuff you say about blacklisting and so on is correct, and applicable to both PoS and PoW.

When selling coins before the double spend, you sap the network of its value by tanking the price right before you try to extort it for value.  You can argue about the economics of this, but I do think you are missing my point about selling the hardware after attacking a PoW coin.  In either model you can recover some of the liquidity used to attack the chain.

As for the hard checkpoints, you're right that those are done by the devs.  I'll get back to you on whether or not there's a reorg depth limit.