1. Not many people understand the afterlife while in the flesh, but all you need to know is that it is possible for all to ascend higher. Eternal punishment doesn't exist.
2. It's made of consciousness and the soul itself is invisible. It is the spirit body which is discernible, which contains the soul. If I had to stick with matter or energy, I would say it is closer to energy(even though matter IS energy). It's a bad question though.
3. Nothing burns for eternity unless stars are reignited or something. The "hells" are multiple planes on the first Spiritual Sphere, though the average person arrives on a plane that is similar to earth. The hells are all about darkness, the worst area being pitch black. The place represents its inhabitants who don't posses a bright spirit. The light of a spirit usually represents its spiritual progress. When people die, they go to the place they fit in best concerning their spiritual advancement (known as the Law of Attraction). As I said before, it is always possible for one to reach a higher level.
4. I've only read of one example of fire in one of the hells. It was a massive volcano-like structure, bigger than what we have on earth. But this was an object created by the powerful will of one of the inhabitants, a creation of thought. In a material sense, I don't think it would have a temperature.