Can the forked be unforked? So: BTC, BCH, BTG and BSV all trade at different prices/values.

All have forked off from BTC and have different development teams, theories, practices, technology, etc.

I noticed that when you look at the amount of coins mined on sites like coinmarketcap.com, it shows the following circulating supplies for each:

BTC = 18,287,125
BCH = 18,351,063
BSV = 18,348,240
BTG = 17,513,924

Are the amounts of the circulating supply of coins (other than BTC) the amounts of coins mined after they forked from BTC?

How is BTG's supply calculated since it is lower than BTC?

Also, if/when one of these coins "wins" through having the "best" technology, most TPS, etc., will they all converge back into one coin some day possibly?  If/when that happens, will the values all become the same since they will converge back into one coin?

Any light anyone could shed on this would be GREATLY appreciated!
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.