On the topic of the scrims, I believe that they were in place to "blur the focus" of the ghosts and scenery, suggesting a haziness that isn't of course actually in the room. I don't think they were so much implimented to be used as a screen for projections of fog, if that's what's being implied by whomever. They were just meant to throw off the focus of the figures by placing an out of focus object in front of them.
For example, when you look out a screened-in window at a tree (or building, or car, or whatever) you're looking beyond the screen at the subject. Your eyes focus on the tree in the background, thereby causing a layer of "fuzz" in the foreground from the unfocused-on screen. Of course, you can choose to focus on the screen or ignore it because you know it's there.
I think that's the trouble with fans like us. We know the scrims are there, so we see them easier. I imagine that another guest who was unaware wouldn't be so apt to focus in on them, being that they don't know there's anything to focus on. I could be wrong, but that might just be me. I doubt the Imagineers planned on anyone knowing how the trick was done back in the 1960's, as it seems to be that sort of supplementary illusion that enhances the scene and isn't the focus. "There's so much craziness going on in the graveyard, who would pay attention to the scrims? That's probably the least-interesting illusion in the whole attraction, who would care"?
We would . . .