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 ) ?
Yes, the halving is correct but not the sums, the block reward is 6.25
BTC per block and will be halved to 3.125 per block
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, but you can check in some back alleys, maybe you will get an old cat lady with a more accurate crystal ball.
3. Will BlackRock and the central bankers be able to fork BTC into a proof of stake coin? ( How are changes to BTC created ?)
Anyone can fork Bitcoin and have their own version, you can do it also if you have basic knowledge.
There is already a Bitcoin Pos fork:
https://www.bitcoinpos.net/