Alternate explanation: toxis gas from the coal fires under the city

I was really hoping that the movie would have a relatively simple explanation for things. There was a hint part way through about coal fires making the town uninhabitable, and why people left. I slowly formed a narrative that would have been an enjoyable twist ending.

The fires were releasing gases that were poisonous to breathe. The concentration changed over the day. The warning alarm would sound when the concentration was overly high, when people needed to get into a place with safe air to breathe. That's always when she would start to black out and the monsters would come from the walls. When the monsters would dissolve, it looked almost like coal dust glowing and floating off.

Throughout there are people in gas masks. Midway there are three people in gas masks in the building, and they have what looks like a canary in a cage. Suddenly they see the bird agitated and immediately leave the building, as if it were detecting the gas.

The religious people started the fires when they were doing the sacrifice, as it got through into the coal mines. The church they retreated to was well-sealed so that they weren't affected when the gas concentration got too high outside.

Then later they send her way down underground to that room, which would be really close to the mines. I figured maybe there she'd find some way to put out the fires, and that would solve the issue, and then the religious people would spin it.

This would have been one of those satisfying endings because everything was explained as a natural consequence of something simple.

I haven't played the games, so I watched this movie on its own terms.