4. History attack - attacker can buy whale's private key for $5 and build alternative story. Solved with some checkpoints now, located behind max rollback possible, so the solution is not so scary in terms of centralization etc.
HOW is that solved??? Centralized checkpoints = not decentralized currency.
rolling checkpoints are not centralized
So where's the whitepaper on how you created decentralized checkpoints?
The network won't accept reorgs deeper than 720 blocks so block 721 back from the current block is the rolling checkpoint. That's how it is done, though there isnt a whitepaper.
There is a general Nxt whitepaper, I can get the link if you haven't seen it.
Kind of hard to keep up with what exactly NXT is and whether it works or not:
As far as I know at least the first version of NXT's PoS is a direct clone of PPC's with some modifications, appeared lacking a good understanding of the security involved in PPC's PoS.