Dispater does not technically have divine ranks, he has an Ex ability granting him divine ranks, but I can see where you are coming from.
I doubt you can turn into things with divine ranks. That would essentially allow you to have a bunch of level 21 characters turning themselves into Dispaters, or Rules of Three or whatnot. Just my two cents.
I can by RAW. That's one of the reasons Epic Spells are broken. A bunch of 21 level characters would suffer from the fact that their HD isn't high enough to get Dispaters abilities, my HD will be high enough to get it.
You can always have a party of four anyway. The number of divine ranks is limited to 18 in total, but you could have a DvR 18 deity and three minions.
Sure, but with leadership and enough charisma I could have level 20 followers, who could be thrallherds commanding around spellcasters to give me a bunch of 9th level spells to fuel divine spells.
Those minions wouldn't be worth bringing to the plane, but having a lot of permanent epic spells could be handy.
Also, my minions could be used for researching the epic spells so I don't have to waste the negligible amount of Exp myself.
The problem with this challenge is, that with no way of shutting down the Iron Tower, or Teleporting within it, it becomes technically impossible just moving within infinite space, I mean, Dispaters foyer could be the size of the universe times three, so just getting past his welcome mat is technically impossible without teleportation, since Deities can't circumvent this with a rank check, the only way to get this is to get a token that leads to the throne room, which none of them probably does.
I'm also assuming Meorem doesn't have access to his throne room because it would be stupid, and since Dispater is probably as paranoid against him, even taking his place and being Lawful Evil (so he doesn't recognize the change) would be a fruitless effort.
Immune to mind-affecting abilities.
So if I have a way around this immunity, such as being a dread witch that adds the fear descriptor to my spells, I could do this and mind rape him.
The fact that I can then summon him, and affect him with mind affecting spells makes it more interesting, I could summon him, mind rape him, compel him to planeshift back to his throne room and do nothing, then set a trigger that when I arrive in his throne room he is to commit suicide.
If I really wanted to get into his throne room, I'd guess that Foresee + Transport seeds would work.
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Also, would the SDA Divine Recall, Teleportation (or better yet, movement) allow me to remember every teleportation spell ever cast. Meaning I would be able to find Dispater by simply remembering where he teleports?
EDIT: How about the control creatures SDA? It is not a mind affecting ability, but it allows you to control all creatures within the same plane unless they save, then I just need to buff the save DC to 82 so I can make Dispater come and do whatever I tell him. I guess an infinite number of devils would turn up, but I guess it would be solved by having them all kill themselves.