if the consensus is considered as one of the benchmarks to support the 21 million Bitcoin numbers, then how can we clearly describe it that until now no one has tried to do this?
It is not. Your full node should have the code that enforces the specific parameters pertaining to Bitcoin's rules. You can verify that you are infact enforcing the 21 million rule by checking the rewards halving, difficulty re targeting and block rewards within the code.
It is easy to change the parameters, whether anyone wants to follow your fork is an entirely different issue.
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. but in essence if the 21 million amount can be manipulated, then would you please give us a scheme to use?
They don't. Seems absurd that you think Bitcoin is within a control of any single entity and is still surviving at this point.