Security of Windows 11 - Offline - with VirtualBox + Whonix-Gateway Using USB Ethernet Adapter: How Safe?
This is unspecific to Whonix.
You could rewrite that question for:
Security of Windows 11 - Offline - with VirtualBox + Debian Using USB Ethernet Adapter: How Safe?
And then ask elsewhere since that is a general computer security question.
This really depends on your threat model. If/how much compromised you consider Windows. If one would classify Windows as totally compromised and actively working against (targeted) users, then as always, the host operating system always beats any virtualizer and any operating system running in a virtualizer.
Also seems a rather useless experiment to me. Best to just drop Windows, use Linux and never look back.