You can't mathematically prove it, because the other stake-holders can roll back to any point of time of their choosing.
That's why I said that PoS 1 != PoS 2 != PoS 3. There is at least one PoS implementation where what you just said is not possible.
The NXT prohibition on deep re-orgs just makes such re-orgs a manual process
if/when they they happen.
Edit: Bitcoin proper now implements something similar called "safe mode". This was implemented after the
last major Blockchain fork