that does not mathematical proof is impossible. also the nxt blockchain cannot be rolled back past 12 hours. for any reason. so instantly you have proven you do not understand the tech and that you do not know how PoS actually works. the white paper and the "math of nxt forging paper" prove that what you are saying is not true..

what you say could be true for other variants of PoS but it does not have a leg to stand on when discussing nxt.

Then you are introducing trust assumptions and new attack vectors. There are no universally trusted parties to provide checkpoints.

It's worth noting that by writing a well-defined security model and working toward it, it is possible to create a "working" PoS which is only broken when the assumptions of the security model are violated. If one were to do this, it would then be easy to point out how the security model is not applicable to the real world. But Vitalik's posts --- and no PoS writeups that I'm aware of --- actually do this.