You're trying to use Plesk in an environment it's not build for. Plesk (or any control panel for that matter) is build for use in an open, connected environment. Running Plesk in a restricted or offline environment undermines it's core functionality, security posture, and maintainability.
I am not saying it's not possible to use Plesk in a restricted environment, but the reason the various repositories and URL endpoints used by Plesk (and extensions) aren't documented is because Plesk is build to be used in an open environment. Because your use case falls outside of what a control panel is typicality build for, your best course of action, imho, would be to map each endpoint and repository yourself.