Can the forked be unforked?
When a fork like this happens, and a new cryptocurrency emerges, the blockchain history from before the fork remains the same.

The amounts you have listed are the coins mined from before the fork + coins mined after the fork. The amount mined will always vary, because of possibly different rules or because the blocks don't always get mined exactly every 10 minutes (BTC block time has jumped up to 15 minutes and will probably slowly decrease over time, while BTG block time is 10min and 26 sec).

BTG supply is lower from BTC simply because from the time BTG forked, more BTC blocks were mined than BTG blocks.

The supply is calculated with the help of total blocks mined and the reward issued for those blocks.

After a fork happens, cryptocurrencies can never merge again. There will forever be separate blockchains.