Windows 10 Offline upgrade

Have you tried to download the iso file and use it to upgrade by running it directly without creating a media?

https://www.microsoft.com/en-us/software-download/windows10ISO

I've done this on a few systems. Using the media creation tool to create an media USB key, and running the upgrade tool from the resulting usb key on a windows install would upgrade the system without asking for the key if the current windows installation is activated. If its not an activated system, then it'll ask for a valid key.

I used the windows 10 media creation tool to download an ISO file from Microsoft I don't recall having to input a license key but I was running from my activated 8.1 copy of windows. It gives you the option to save an ISO. I then used the windows 7 usb/dvd download tool to install the ISO on a USB stick. I then just ran the setup file from the usb stick to install windows 10 as an upgrade on my Laptop.

If you do have a running Windows 10 machine in your LAN, you could also use the function provided with Windows, that each pc shares his upgrades with either machines in the same LAN or also in the Internet. So first install Windows 10 via windows update and gwx.exe on one machine and then the other machines should see that win10 machine and download the updates from that instead of the Windows update server.

for more information about it see this article (ger/de)