1) Why would we do that. There's no reason to fight a completely benevolent being just for shits and giggles
2) Yes she did
3) Yes he does. He gets it after using "Start From Scratch"
4) We saw Tartarus in a cutscene, it isn't a physical plane of existence as we'd understand it. Teodor also described it almost completely.
5) Take a guess.
6) He was always Hades. Hades wasn't possessing anyone, Hades was a creature born of a man's ambition. The man was Melvien (or whatever his name was before he discovered Tartarus). Again, this was explained in detail.
7) If you watched the final cutscene, you have seen that they weren't just sitting there in the basement watching the lifeless corpse of Hades.
8) I knew this was coming. He wasn't a Zilart, Zilarts had nothing to do with this. At all. Not only is the man we know as Melvein clearly an elf, but you can see that the man that was Hades before he was called Hades was also an elf very clearly in one of the cutscenes (further clarifying that Melvein is, indeed, the same guy with a different name). He clearly states that Ra'Kaznars existence is a testament to his power/longevity, implying that he, i.e., an elvaan, not a zilart, built it. Odin had nothing to do with it either, we already know where he's from. Tartarus is as of now an unknown in terms of whence it came, Hades merely stumbled upon it and used it to his own ends by using his pawn, Teodor, to subjugate those "imprisoned" or "inhabiting" the realm. From the sounds, Tartarus is more of a prison for demons and beings of great malevolent power, or perhaps even those of benevolent power who have made some god or other entity fear them (i.e., the dragon).