→ Root Directory key in payload parameter.
→ Generate invalid payload in Create service template Create service
→ Connect private / public GitHub repository
The monorepo/root directory feature is recently added and has not yet been added to the Public API, but it’s certainly on the list.
I’m not quite sure what your other points are referring to. You should be able to use public or private GitHub repositories (as long as your account is linked to a GitHub account with permission those repos).