1: I think you can, but I'm not sure

2: No, Raiden and Odin are the same Esper, you can't get them both

3: No, You're screwed

4: You can get two Atma Weapons in the game... One in that cave to the Sealed Gate, and you can steal a second one in the 3rd stage of the Final Boss

5: Weapons and Armor: Give them the best stuff you might have... Minerva, Atma Weapon, Ragnarok, Gengi Shield, Thornlet...
Give the Gem Box and a Economizer to your best mage, and a Gengi Glove and the Offering to your best fighter

And No, there are no special ending, but you can see more cutscenes depending on how much characters you got...