I have not read The Shack and won't, nor have I viewed the movie and have no intention of watching it either. From what I have learned about it the Shack, has some blasphemous portrayals of the godhead as you pointed out. C.S. Lewis is not one to look to for good theology certainly but he is a wonderful writer and his works of fiction, as long as you do not draw from it any theological conclusions, are great. I have read a lot of Lewis over the years and enjoyed them very much. His theology has been dealt with in previous threads which may help answer your question.

The Chestnut Mare