You are right on target in what you have to say about painterly representations of the crucifixion and their relevance to the film. Grünewald is of course a closer analogue to what Gibson is trying to do than Cranach, and since our last exchange, I see that David Denby has noted the Grünewald connection in The New Yorker. Also, I'm fascinated by your mention of the tableaux vivants of the passion plays. This is a correspondence that no one else, as far as I know, has identified, and I am struck by it because it helps locate precisely what is wrong with the film. Let me hasten to say that we needn't assume that Gibson and his collaborator were actually aware of tableaux vivants of the passion plays. The point is that in their rather self-conscious effort to produce a cinematic equivalent of the traditional iconography of the Christ story, they ended up arranging the principal actors in poses over which the camera could linger that end up producing an effect akin to tableaux vivants.
We remain in disagreement on the success of the figure of Mary in the film. Maia Morgenstern's silent acting is indeed finely executed, but because this is not, after all, a silent film, it was not enough to engage my feelings deeply as I watched the movie. Then again, the extravagant hoopla before the release of the film, reinforced by the bludgeoning to which I was exposed as I watched it, may have led me to sit in the movie house with a heart of stone.
About Judas and Pontius Pilate, on the other hand, I agree with you. Judas is humanly interesting because he is shown overcome with remorse after the betrayal, actually trying to revoke it, and the depiction of the way he hangs himself is almost understated—certainly for this film. (I suspect that Gibson wanted to reserve all the gruesome frontal suffering for Jesus.) In any case, Judas has less than five minutes on screen in a film that lasts more than two hours. Pilate is, as you say, the one other figure who holds some interest as a character, but the problem is that his relative complexity (in rather brief episodic appearances in the film) reinforces what you worry about as its possibly "nefarious" aspects. If this were, let us say, a historical novel, a writer might well want to flesh out the minimal notations of the character of Pilate in the Gospels by turning him into a reflective man dutiful but ambivalent in his political role, a cultivated Roman who finds himself saddled with a rabble of Aramaic-speaking Asiatic natives and who is trying to make the best of it. But this is obviously not a film about imperialism and its discontents. In the raw, insistent "sermon"—I like the label you give it—that Gibson has tried to make, the characterization of the Roman governor as thoughtful, perhaps even humane, has the effect of making the bloodthirsty Jews seem all the more savage, and that pushes the representation of the Christ-killers further in the direction, as you aptly put it, "where the muddle of the movie matters."
I have no reliable sense of Gibson's motives, so I prefer to call the film nefarious, or at least much of it, rather than his purposes. Let us assume, as you propose, that he wanted to make a sermon in moving pictures. We all are aware of the commonplace that stained-glass windows are sermons in pictures, and similar comments have been made about much Christian painting. But a long sequence of filmed images, projected on a big screen in a darkened theater, has an effect that neither a verbal nor a pictorial sermon has: It imposes itself on our imagination while we watch the real thing itself; or, to rephrase this as some film theorists have, it sweeps us up into a collective dream that, for the time of dreaming, we accept as though it were reality. Perhaps Gibson truly wanted to convey the "reality" of the sufferings of Christ, and perhaps he merely wanted to make real for audiences the Jews as they are represented in the Gospels, crying out for Jesus' blood, with no thought whatever about Jews in the contemporary world. (It's perfectly possible that, as the saying goes, some of his best friends are Jews.) But even granting him this benefit of the doubt, we may have a problem, as you imply at the end, of unintended consequences. These images in sequence that pound both the body of the crucified one and the sensibility of the viewer are vehemently unsubtle, and in their terrific bluntness, their lack of explanatory context, they could be an ideal vehicle for propaganda, not preaching.