This was not stated in your original post. That is an important point.
Good advice, I will do it.
If your system is based purely on proof-of-stake, what prevents me from executing a sybil and getting 90% of the NXT coins on release?
This hasn't come to my mind... Any ideas how to counteract sybil attack?
Maybe now, when you revealed such problem, someone else will try it too and you both have to compete and this will be too expensive? If 10 people will try to buy a lot of NXT they will get 10% in average...
What prevents people from gaining 51% of stake in NXT if there are no new coins created and there is no mining in this system?
In PoW currency you can
remine a block to build a longer chain. In Nxt the order of generating accounts is determined, you can't create a long chain that contains blocks generated
solely by you. With 51% of the stake the odds to generate a longer chain with 10 blocks are less than 0.1%. If someone buys a car with NXT they can wait a little bit longer to counteract even 90% attack.