[Q] What exactly prevents this script to be usable on Windows 7?

[Kein]

Assuming I have all Win10 required tools to manipulate the image: BCDBoot DISM Oscdimg

separately, what other requirements prevent this to be usable on Win7? I'm not particularly familiar with extensive PowerShell scripting but from what I see the main script mostly manipulates Registry, which you can do via reg and its commands once you mount the WIM image.