It should be legally required to provide enough interoperation capabilities for a compatible frontend to be written for an Apple II by whoever would like to do that, as the government can't be expected to write and maintain clients for every platform that's now in existence or that will be created in future.

If only currently popular platforms are to be supported, how could a new platform join them in the future if the use of existing ones is mandated by governments?

No, but it should be open enough to be reasonably independent of specific services and devices.

Simple, provide a simple API, let the community build the clients for the machines they have.

The problem to solve is trust.

The technical solution is a hardware root of trust. This is typically a specially hardened chip in the device. A Trusted Platform Module (TPM).

Your Apple ][ does not have a TPM. It cannot run software that can assess it's identity in a trusted manner.

You can make an argument without pulling it into the ridiculous, you know?