What if we're going about this the wrong way? What if we consider the previous maps, excluding the first and maybe 3rd map, because the first was an introduction, and Shi No Numa was Japanese so yeah. And let's also note that Richtofen knows what the f is going on, as you can hear in the loading screen. He knows everything concerning the zombie project. Also, as for Cosmic Silverback, maybe it's not him, but if you look on the stage next to the entrance to the dressing room and look in the pods, you see what appears to be human/ape hybrids. As for the shaking, I personally agree with the idea that it's not the Gas Zombies, but I'm not fully convinced that it's linked the supposed boss, supposing there is one. Also, let's try and work out all the absolute 100% undeniable facts about it, then construct into a storyline, then add theories to that and see what we get. Just my piece.
EDIT: I just looked around, and on the end of the stage opposite the teleporter, there is a pod that has fallen over and opened. You can see for yourself. There is something else in there.