Hey Guys , I Have Some question About Bitcoin

1.If There Is Any Hardfork, Who Must Agree? The Miner Or The Nodes?

2.What Is The Benefit Of Having A Lot Of Full Node(For The Network)?

3.Is The Decentralization Depends On Hashrate Spread Or The Nodes Count?

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?

5.Why Did Block 501726 Has Zero Block Reward?

Thanks