As far as your first question, yes, if that code is in functions.php no posts should be saved and all fields should show the error, if not then for some reason the filter is not being called, I don’t know of any reason why that would be the case with ACF activated and that code in your functions file.
In your second piece of code, there isn’t a hook acf/validate_save_post, there is a acf/save_post hook.
