the hidden area behind the wall

That scene is actually my favorite part of the whole movie. I believe the relevance to the scene is to show that there's absolutely no way they are getting out of the theater, and many of the characters hit their breaking point. I enjoy the scene because of the malevolence presented. The building itself has manifested into an evil entity. Afterwards, when they are seen again sitting in the balcony, they appear to have lost all hope of escaping. I don't feel that scene would have been as effective if they hadn't discovered the area behind the wall beforehand.

I always thought that it must've been something in this room that infected the two girls in the end. It's the only way that could possibly explain how they became demons.

At first I thought that room was going to have ancient artifacts or something that were the source of the theater's power. This wasn't the case. However it definitely has an evil presence since there was that demon goo dripping from the wall before they broke in.

Only people with money say that money doesn't matter.

I always got the impression that it was meant to portray the fact that "they" only renovated the parts of the cinema that they needed to stage the movie.

Leaving an entire section of a building in that state doesn't really matter if everyone dies via demon attack.

I believe I read it here a long time ago that the theater used to be a huge church. They basically just converted the inside into a theater. Everything else was blocked up. This would have gone well with the original origin story part 3 would have told. Sadly that never got made and instead we got The Church.

In The Church, witches are executed and buried under holy seals, Churches were then build over these seals to prevent them from breaking and allowing the evil spirits from within to escape. What if the original script had these people as being possessed by demons instead of witches. Later in the movie, the seal breaks and people become demonically influenced instead of possessed. I can only imagine the original script had them all being demons and going around a church killing people. Because the seal is broken, the church goes into lockdown. Basically the same thing that happens in the House on Haunted Hill remake happens in the movie. No one can escape. Probably in the original script, the church seals up supernaturally like the theater and the apartment did as well.

the movie also had Asia Argento in it. Maybe she was supposed to play the same character from part 2. The last time we see her, she's stuck in a car surrounded by demons. Maybe she got away. She plays the daughter of the caretakers of the church and is the only one who knows of a secret passage leading in and out of the place.

I wondered if maybe that was a gateway to another dimension. The movie tends to shift at that point and everyone is a demon.

I often have trouble understanding why some scenes exist in these films. Seems thing to jump or cut erratically, with characters exchanging roles of importance, being introduced, disappearing, so rapidly it almost doesn't feel like a story at all.

Per example, I always thought the red headed concierge girl was going to have some sort of impact... Instead she just becomes one of the "survivors" and at some unknown point a demon.