I may not have ever worked for Disney, but I do know how scrims work. For someone who works (worked?) for Disney, your knowledge is surprisingly WRONG.

At no point in the Haunted Mansion do they use scrims, except in the ceiling of the stretching room. A scrim CANNOT be partially lit. It is either fully invisible or fully opaque. In other words, you can either see everything through it or you can nothing through it. Let's take each of your "points" one at a time. Granted, if part of the scrim is painted, that part will still be able to reflect light when the back of the scrim is lit, but the part that is "see through" can only ever be fully opaque or fully transparent.

The portraits in the hall of portraits use a pigment that reacts to a certain frequency of light. When the lightning flashes it causes that pigment to glow in reaction to the light. This is why you can see the original portrait while also seeing the gruesome image AT THE SAME TIME. If it were a scrim, the lightning would have to come from THE BACK of the canvas, not the front. Plus, the paintings would obscure anything you might see from behind. You can see the lightning flashes on the wall around the portrait which clearly means it is coming from the front. Thus, NO SCRIM.

The endless hallway does not have a scrim because it does not need one. The hallway is long enough to really not even need the mirror. Simply lighting it properly will give the illusion that it is endless just by the fact the guests can't see the end of it. Further, the candelabra does not need a scrim to make it appear to float as fishing line is sufficient to achieve this effect. With a bit of gently wind from a hidden fan the candelabra can appear even spookier as the wind makes it move about on the fishing line, especially if the line is anchored at the top and bottom with just a little slack. As for not ruining the effect in the mirror, if the candelabra is in front of the front mirror (two mirrors are used to achieve the effect of an endless hallway), it will not be reflected by the mirrors. Finally, there is no need to obscure the mirrors with a scrim as the only lighting is coming from the chandeliers and the candelabra. This means that the effect works perfectly without needing to obscure it in any way.

The graveyard scene might use a scrim except for the fact that artificial fog is not difficult. Plus, creating fog is far more effective at making a scene look foggy. Considering the entire room is dimly lit, I very much doubt anyone would see the mechanisms that create the various ghostly effects. Plus, the guests are not stupid enough to think the ghosts are real no matter what you do to hide the various metal bits. If you bother to ride or watch video of someone riding, you will notice that many of the "ghosts" are clearly animatronics as they are not moving in anything remotely resembling a natural manner. Their movements are mechanical and jerky. Also, the graveyard is curved. Trying to put a scrim between the guests and the scene would be difficult at best. It would be necessary to ensure that the scrim does not move or sway in any manner. If the scrim moves, it ruins the effect. Thus it would be MUCH easier for the entire scene to be blanketed in actually albeit artificial fog.

The only place that actually uses a scrim, as I said before, is the ceiling of the stretching room. You didn't even mention that! When the lightning flashes ABOVE the ceiling in the stretching room, the guests can see the corpse hanging from the rafters. They CAN'T see the ceiling any more. THAT is how a scrim works. There is no other place in the entire ride where that effect is seen. Honestly, I'm surprised you didn't try to say that the hitchhiking ghosts that ride for a moment with the guests at the end of the ride are using a scrim.

The wire saw is broken and the water drops are worn