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)
They do implicitly. While nodes don't check the mempool for ongoing double-spend attempts they won't accept blocks that have double-spends in them (ie. blocks that try to spend inputs that have already been spent are invalid).
fair enough.
i was talking about double spend attempts on low/no-confirmation transactions, but you're correct. naturally, full nodes will reject tx that try to re-spend outputs already confirmed as spent.