Re: No Windows 10, no Office 2019
"The last time I installed Office 2016 for someone (1 month ago), I used the license key and it made me create an Office account before it would activate."
No such issues here. I have used both MSDN and MAK keys with Office 2016 and you can definitely activate it with no account.
Microsoft say thats still the case with Office 2019 too:
Office 2019 uses device-based licenses similar to previous major versions. Users will be able to sign in if they want to. However, they are not required to do this. Key Management Service (KMS) and Multiple Activation Keys (MAK) activation are available to volume license users for offline licensing. No internet connection is required to use the product.