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Hyper-V Server 2019 (Free) as a Virtual Machine and Application Server
Legal licensing issues cannot be resolved by anonymous forums users. Your lawyers would not be happy with that. The best we can do is point you towards documentation. Your lawyers may be able to provide a legal interpretation, otherwise you could contact microsoft directly.
Might need to check the documentation for host hypervisor as to nesting compatibility. As to SQL Express you can check the operating systems compatibility here.
SQL Server 2019: Hardware & software requirements - SQL Server | Microsoft Learn
The hardware and software requirements contain information that you can install SQL Server 2019 on Windows Server Core 2016, 2019, 2022. Hyper-V Server 2019 (Free) is a Server Core 2019 with the Hyper-V role preinstalled. My tests confirm this, installation from the command line and configuration in powershell is all that allows you to run SQL without problems.
Hyper-V Server 2019 (Free) is described as an operating system that is often designed to reduce company costs. Everywhere there is information that it is a free and activated system and does not require additional activations. The EULA included with Hyper-V Server 2019 (Free) does not contain limits.
If anyone knows where to find documentation describing the limits, I would be very grateful for the help.