It's just that the issuance of coins is not subject to a central authority, but requires a supermajority consensus.
Yes and no. A bitcoin node sends and receives information, but most importantly verifies. Note that each one runs
autonomously. By that you understand that no one forces none to change a consensus rule,
like the 21,000,000 coins. The already existent nodes define that Bitcoin works that way, since January of 2009. Every bitcoin node follows the rules chosen by satoshi. Everyone is free to change the consensus rules and continue/restart the blockchain, but that wouldn't be Bitcoin's. It would be a hard fork, a chain with different rules.
To answer you, if the majority of the nodes decide to follow different rules, they immediately stop being the majority. But since it's a majority of users (which means that they're a lot), they can manipulate people that
their Bitcoin is Bitcoin.
Whether with the amount of 21 million or not, bitcoin is beyond the control of any party, except for China, which seems to have the capacity to try to change the consensus system and make reforms.
What does China have to do with the consensus rules of Bitcoin? The capacity has nothing to do with the consensus, not to mention that China isn't a single entity to perform 51% attack
if we assume that they have enough computational power in total.