If after closing the grave it was realized that the mayyit was not placed facing the Qiblah, should the grave be opened for rectifying this error?

Answer

It is not permissible to open the grave for this purpose. Just make dua for the maghfirat of the mayyit. It is indeed a fearful sign that the mayyit was buried without facing the Qiblah. This error was induced by Allah Ta'ala.