A ruptured yolk sac looks like raw yolk. Not cottage cheesy. At least that's how it looked when I saw one in my incubator.
Still, the yolk sounds like the most likely candidate if you are seeing a lot of yellowish wettish stuff.
Icky-stuff guy doesn't sound too good :-(
Backwards pipping can often end well - don't rush in to help that one.
Could a baby possibly poop meconium in the shell? It looks more like bird poo to me than anything. Either way, baby has eyes open and peeps from time to time but seems exhausted (as I am).
Backwards guy has just a tiny hole and I've pushed back the membrane enough so it can breathe. Going to stay pretty hands-off with that one for a while.
I'm going up one last time and then I really have to go to bed. Starting to get a little dumb, which means I need to sleep!