1. Is it correct to assume that each halving will result in a 50% reduction in rewards?
Yes and that's the exact situation as it has always been, you received half ofbthe previous reward of the last halving and so it continues till all the blocks have been completely mined.
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 ?
If you're investing on bitcoin, know that this is not a daily income generating business or scheme, you are rather investing on a volatile digital cryptocurrency called bitcoin, whereby during this your period of investment, you're expected to hodl while there will be bear and bull markets during this time till we finally arrived at bitcoin Halving where the market surges and there comes a new all time high and your investment would have been very fruitful.
3. Will BlackRock and the central bankers be able to fork BTC into a proof of stake coin?
No
( How are changes to BTC created ?)
Through BIPs