Turns out that the easiest way to download a FULL package of npm is not via its GitHub repository, but by downloading Node.JS' source archive release!
inefficient / incomplete npm downloads
Download methods:
The release archive from npm's GitHub repo does not contain some of its dependencies. Thus, downloading that archive may be fast, but unusable!
npm install -g npm@latest -> This is not ideal as it's slow! Since it needs to download the countless dependencies one at a time. Sometimes, npm also messes up.
Complete npm download in one step
The complete npm with all dependencies is available in Node.JS' release source archive =)
Simply:
Download the desired release archive (.zip or .tar.gz) from Node.JS' release page.
Extract the entire npm folder from the downloaded archive. It's located in sub-folder: deps/npm
Yes, the archive includes lots of other stuff irrelevant to npm. Still, it's a single file and much easier to download intact.
QED.