1. Is it correct to assume that each halving will result in a 50% reduction in rewards?
( If current reward is .0003 BTC, reward after halving will be .00015 ) ?
Not assumption because it is hard coded in the Bitcoin Protocol. Bitcoin block rewards will be halved after every 210,000 blocks.
Controlled SupplyHow is the 21 Million Bitcoin Cap Defined and Enforced?2. Is there a good calculator for ROI and estimated rewards ( per day ) that calculates 5-10 years in the future or allows an input for estimated future value of BTC ?
No estimation for ROI for miners because it will depend on their cost to mine bitcoins and price of Bitcoin at the same time.
Equivalent network time is interesting. It shows how many time miners will have to spend more to mine 1 bitcoin with time. It's not ROI estimation.
3. Will BlackRock and the central bankers be able to fork BTC into a proof of stake coin? ( How are changes to BTC created ?)
They can fork like hundreds of useless forks and many of them died.
How Many Bitcoin Forks Are There? You will be surprised!!!