1. Is it possible, 100 (or a certain number) billionaires buy 90% of the total circulation? If it is possible then 2. Will Bitcoin be able to maintain its decentralized position? Or will it be played by a small number of people like current fiat currencies?
Will you sell your BTC, if you hold any? I hold some Bitcoin, and some holders have been holding BTC for a long time. What if they don't plan to sell? So theoretically your calculation is right, but practically it is not possible because not every bitcoin holder will agree to sell even if you convince all the billionaires to buy it.
There must be 100 billionaires or maybe more who have bought BTC but the amount you are asking is too big for all of them. They won't invest such a huge amount in one asset and that's their main lesson to become a millionaire. The answer to 2nd question is, the integrity of BTC in terms of decentralization won't be sustained if fewer people own more BTC, they can easily manipulate the price.