1.If There Is Any Hardfork, Who Must Agree? The Miner Or The Nodes?
in a successful hard fork, all nodes must upgrade. if users don't agree, there will be a chain split. so miners can try to hard fork, but if users don't follow, miners will be forced to return to the original chain out of profit motive.
2.What Is The Benefit Of Having A Lot Of Full Node(For The Network)?
full nodes force miners to be honest. they do this by enforcing the consensus rules. if no one was validating transactions except miners, miners could violate the rules---for example, they could inflate the supply beyond 21m bitcoins. SPV users would never know.
3.Is The Decentralization Depends On Hashrate Spread Or The Nodes Count?
both.
4.When Node Are Verifying A Block, Is it verifying (Checking For Double-Spend) it From The Whole Blockchain? If That Right, Is That Mean The Node Requirement Will Be Higher In The Future?
nodes aren't checking for double spends. (unless their operators add code to do so)
yes, full nodes verify the whole blockchain. this is the only way to verify proof of work. requirements to run a full node will continue increasing, but storage space is a minor concern since it's becoming so cheap.
5.Why Did Block
501726 Has Zero Block Reward?
i'm really not sure. that's a strange one.....