Couldn't the tableaux-vivants be a purely cinematic device? That is, should we see them as having been staged by the director of the movie to illustrate the collector's analysis of the paintings? That doesn't seem to be Ruiz's intention. He shows the figures breaking character occasionally, a woman imitating the goddess Diana, for instance, taking a brief break from her pose before returning to reassume her position. Were she merely a cinematic device she wouldn't have to go anywhere. In a way, of course, she still is a cinematic device, but now she signifies not just the figure in the painting but some subject either of the collector or the director, someone recruited to put on a costume and strike a pose in a garden for as long as the intellectuals need her to. There's nothing necessarily sinister about that, but there is something vaguely decadent about it that's in keeping with the tone of the film.
For what it's worth, the hypothesis of the stolen painting is that the collector's Tonnerre set isn't complete. It follows from his core thesis that the existing paintings form a sequence, one linked to another by certain symbols and effects. In the Diana painting, for example, a character holds a mirror to reflect sunlight. The collector deduces that this reflection accounts for the otherwise-impossible multiple light sources illuminating a room in the presumed second painting of the series. Each painting in the series is referred to in some way by the next one. This sequencing convinces the collector that their must have been another painting, lost or stolen -- confiscated by the authorities, perhaps -- that had a mask as a design element. Further, after an intense analysis of a specifically narrative painting recounting a family scandal, the collector traces specific gestures that recur in all the paintings, ascribing to them ritual significance in a cult of Baphomet, the archetypal androgyne.
Maybe I missed something, but couldn't that be the stolen painting in the background of this tableau-vivant?
There's something unsettlingly excessive about the story -- not the idea of the Baphomet cult, but the idea of considerable mental and artistic resources dedicated to the solution of a fake mystery. Again, however, nearly all movie mysteries are fake, yet their development and resolution rarely compels us to question, as Ruiz may want, why we -- artists and audience alike -- dedicate so much attention to such fictions. I'm pretty sure Ruiz and Klossowski know what they're doing here, and that they mean us to be as disturbed by the indulgence of the collector's inquiries as by the scandals he recounts. But other viewers may interpret the film differently. The filmmakers succeeded in making the film itself a mystery, and rather than being annoyed by the apparent irrelevance of the mystery of the paintings, I was captivated by the self-consciously provocative contrivance of it all. That Hypothesis challenges you to find a point to it may not seem like a recommendation, but the way it seems to invite critical thinking is one.