If you do fall through, there is a ladder back up in the back-left of the room. I too thought that falling down and killing all the wretches was part of the game but it is impossible, there are far too many!
I don't think it matters if you fall through because eventually you will find the way that doesn't break and it is easy to run back to the ladder when you know where it is!!
Completed the game on hardcore now, and think I'm ok online now. We should get some TB Gears of War on Live sometime, or anyone got PES as its on its way to my door as I post and will be well up for some games on that once I am used to it compared to PS2.