You do it from a Windows computer that Microsoft assumes you already have access to for the purchase. MS is targeting PC owners here, not Mac users in a VM environment. That doesn't mean that you cannot use it in said environment, but MS just isn't making things convenient for them.
Yes it's a bit arrogant, but that's how MS chooses to do things.
I don't understand the comment at all.
If you work from the VM, all is fine. You download the stuff from the VM, and run it like you would from a non-emulated Windows environment. The MS installer has no idea that there's an Apple underneath.
If the download is an ISO, you put it on the host OS and attach the ISO to the emulated DVD on the guest, and install normally.
So, the new ISO download option is for both Apples and PCs that are doing bare metal installs, not running the installer from Windows.