Can Microsoft Office for Windows key be used for Mac Office version?

It’s possible, I have done it. The licenses are platform-independent. All that’s needed is to uninstall and then deauthorize the existing machine on the Microsoft account page before installing on another.

That MS article does not make it clear if a cross-platform transfer is possible. The Mac and Windows versions of Office are coded differently and have different features and user interfaces. You would have to download a Mac version and only MS can tell you if that is possible without buying a new copy.

I believe what you want o do is possible with Office 365, but its licensing is different.

That is a question for Microsoft.

Yes, see https://support.microsoft.com/en-us/office/transfer-your-office-license-to-another-device-or-another-person-8a967fb6-6c65-433e-800e-b9ae3436c2de