Office for Mac also requires the installation of “Serializer”. You can skip it if you have the serializer installed.

See the JUSTSYSTEMS website. (in Japanese)

See “Choose between the 64-bit or 32-bit version of Office” (link to Microsoft site).

This means that the OS can be upgraded (changed to a higher version) only if it is already running a Windows OS.
If your PC already has a Windows OS when you buy it, you are eligible.
In the case of a Mac, the upgrade can be applied as an OS that runs on virtual environment software or boot camp that can run on a Mac.
Downgrading the OS is also possible, but not recommended.

A “new install” installs a new Windows OS.
In contrast, an “upgrade install” inherits the settings of the original Windows OS.

You cannot install unless you have a license that you can upgrade from.

You need to separately obtain an OS to upgrade from. The upgrade version can be used if you have a package license that allows installation of a new OS.

No, you cannot.

Yes. Please check this table.
Reference: Windows 10 upgrade paths (link to Microsoft site)

Regarding virtualization at the client OS, up to four virtual OSEs are allowed per license per terms of the Software Assurance (SA) privilege.