Clean Air Hints

I’ve been playing Clean Air lately (and you can too here) and I’ve been really enjoying it and its great atmosphere, but unfortunately, I can’t figure out how to progress. I don’t know if I’m missing something super-obvious (or if anyone else here has played this), but I’m stuck on what I’m supposed to do now. I’ve the combined key, the towel, the hook, the chunk of meat, the letter, and the third thin key, but I don’t know how I’m supposed to clean the marks on the floor of the dead end to pacify It, how I can survive the tunnel down (if at all), what if anything I can do about the Clock, or how I might be able to unlock apartment 202. Any help would be greatly appreciated.