I still don't see how a nothing at stake attack could succeed. Buying majority of PoS coin supply isn't exactly nothing, and finding private keys of the initial stakeholders does not help if you have checkpoints.

Nxt uses a reorg window of 720 blocks. blocks older than that won't be accepted by any client. This means that checkpoints are set up in decentralized manner (each client sets its own reorg limit). You need to effectively buy 51%.

I would like to see a nothing at stake attack succeed, so far I only saw 51% attacks on low hashrate PoW coins. Also I don't see how decentralized consensus should not be possible in PoS? I see it working in real world while the "consensus is not possible" statement is theoretical.