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 ) ?
It is per block not per amount as what others have explained, the reward block is the one that gets reduced.
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 ?
Well, the thing on this one is that there's no accurate calculator depending on many factors if you happen to operate your own miner. Too many to consider just to get to the ROI. It sounds interesting when you see the calculation and projections on how quick the ROI and high the profit may be but all of those are just projections.
3. Will BlackRock and the central bankers be able to fork BTC into a proof of stake coin? ( How are changes to BTC created ?)
Just like with what Ver did and made his own bitcoin cash as a bitcoin fork and we've seen the bitcoin SV, so they're also able to do the fork but it won't directly have any code affection to the actual and real bitcoin.