Training scene where they shove a shower hose down a toilet and use it to breathe...
There would be no air (or even sewer gas) to breath in that case. Toilets work by raising the water level in the bowl above the water level in the S-bend/siphon. Since the room was full of water, those toilets would have been flushing constantly, and the whole pipe would be full of water.
Better(ish) solution. Use the body bags that they each had to fill out and place in their trunk/locker to capture an air bubble. That would at least give you some time to attack the door, or figure out how to drain the room.
It was enough water to fill that room in a few minutes, I'd say a few toilets and shower drains would be a negligible effect. Although once the water got a few feet over the bowl, or other drain, I'd stay the fuck away from them because of potential delta-P situations.
I wouldn't know. There are toilets in the US that use a high pressure flush, but they still have a siphon. It's there so that the toilet can flush without water pressure from the mains (you can flush a toilet by dumping a bucket of water in it) and it also forms a seal that prevents sewer gas from flowing back up through the toilet. So even if the toilet uses an alternate flushing method, it probably still has a siphon.