• I am trying to modify a built-in resolution, but macOS doesn't accept it
    If you are trying to add a custom resolution identical to the original, forget it: you can't.
    macOS doesn't let you add identical resolutions (same width, same height, same frequency).
    You may be successful if you just change the frequency a little bit: from 60Hz to 59Hz for example.
    This doesn't guarantee that it will work, it just guarantees that at least macOS will try to use it.