I've always felt the opposite way about Shirly MacLaine, as an actress and in general. And in this movie, she "played against type" really well, as this "WASP" socialite character. (She even had some funny moments doing it.)
I do agree with you about that feeling of authenticity during the exorcism scene. I know nothing about real exorcisms, including this traditional Puerto Rican kind, but it definitely FELT real.