Was the ending of Mawaru Penguindrum completely incoherent?
The place that Himari goes to (with her two brothers) is called the Crystal World. It's a fantasy world that they get transported to by the Princess of Crystal, the entity that possesses Himari through the hat. Not sure if it can be considered a hallucination (if it is, it's definitely shared between its occupants), but it's probably not in the physical world, perhaps in a "dream" world?

The three penguins were assigned by the Princess of the Crystal to Himari, Shoma and Kanba to help find the Penguindrum. They're not hallucinations since they physically interact with the real world. The reason why only the 3 who were assigned penguins could see them isn't explained.

As for B, I don't think you can look at any of this as "godlike" or "cosmic power". This is purely fantasy so it doesn't make sense to apply the series in a real-world like setting. These powers are fantastic beyond explanation and the entire series is heavily laden with symbolism.


  The Princess of the Crystal wants the Penguindrum and in return, Himari can be saved. At the end of the series, it's revealed that the Penguindrum was actually the apple that Kanba and Shoma shared while in cages when they were little (the "fruit of fate"). So the two boys give up their "fate" to the Princess of the Crystal, thus giving up their entire existence, resulting in a Himari that is alive and well in a world where the two boys never existed. The sacrifice was giving up their fate to the Princess of the Crystal, who isn't a god or cosmic power, but turns out is part of Momoka Oginome's soul, Ringo's older sister and the original owner of the diary (she died from that terrorist attack). Her soul was split into two and became the two penguin hats, one of them being the one that Himari was given.