You do not start in Dis, you start at the gate in front of it. Nothing has ever been said about not being allowed to leave the plane the moment the challenge begins. I fail to see the restriction.
Use the normal rules for deific realms given in the Gates of Hell, chapter 9.
The Iron Siege is not supposed to be fair. You are supposed to fight a guy whose CR is officially just over 60, but you are granted a level 70 deity, that says something. Anyone could make a level 20 character and one shot Dispater, but his extended abilities are what actually turn the Iron Siege into a real challenge. I do not recommend entering the challenge unless you understand what you are up against.
As to Dispater's resources: You have no idea what I, Witch and Selah have used so far. We might or might not have used common devils, archdevils, Dukes or some weird psionic combination, but before making any plans the player should actually see how much real information about my methods he has.
So far I am relatively surprised to see how many very lucrative tactics haven't seen daylight in the challenge.