These witnesses are selected by the users by the voting process. Actually, the vested Steem tokens (Steem Power) plays a very important role in this process.

Yes, that's why it is called Delegated Proof of Stake (DPoS) as it allows everyone that stake on the blockchain to vote for delegates or witnesses who are responsible for data validation and security of the network.

About 3.1 Steem tokens are minted after each new block is produced in the blockchain. So, we can also roughly say that one Steem token is minted or produced in a second.

I wrote about this in my post after I carried out my calculation, I don't know if you also do the same or you assume the amount of Steem tokens minted when new blocks are added on Steem or got it from any online post about Steem.