You can't mathematically prove it, because the other stake-holders can roll back to any point of time of their choosing.
In other words, PoS does not actually form a consensus.
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.