Be careful, the Unity installer that comes with the default download does not have the needed files to install. You can get full installers from the archive.
One good use of backing up the whole engine is for portability. My edu institution, for example, has Unity on its machines, but it is very slow at keeping up to date. One can just plop an up-to-date copy of the engine on an external drive and launch it from there, but one can’t download an install the new version individually because of system security.
No, you’re talking about the download assistant. It downloads the installers and installs Unity, and can optionally be told to keep the installers in your specified path.
Subtle difference. You are suggesting installing Unity of a portable drive. Which is actually a valid option.
Does Unity play nice with such an installation? Some of my software does and some doesn’t.
I do wonder how far you’ll get making games if you can’t distinguish between an installer and a download assistant. If you need offline installation you can download these seperately from the additional downloads link.