I also think the desecrations were carried out by Regan, the book alleges that it was Regan by the clay used to sculpt the obscene appendages on the holy statue, also, in the movie, as Father Karras is offering the holy sacrament in church we see the back of a girl who has a resemblance to Regan sat in one of the pews, could it be that she freely went about her demonic business at will, not to mention the eerie shadow that Kinderman sees pass across her bedroom window, all this from a girl who is supposedly strapped to the bed, where she cannot leave it, or the room.