Is it just me, or is this analogous to the Iron Siege? I.e. some ultra-paranoid being locking itself up in an unassailable plane (cause that's what the Iron Tower is, isn't it). Also, Dispater is played as a PC in a way; he has lots of resources and is immune to diplomacy. Of course, the Iron Siege is different in that we simply
don't know what exactly is up against us, and thus cannot find the optimal way by analysing the conditions, but must use trial-and-error.
I think the reason why this discussion is caster-centric is both because of the mindset and because of reality. Reality says: Paranoid caster isn't going to be beat by anything not substantially more powerful than himself, or specially built to kill the former. The mindset comes into play where everybody tried to beat the ultra-paranoid caster in the first place (at least I understood it that way). Why is nobody trying for the easier picks?
Btw. I just thought about a side-effect of breaking the action economy with Genesis: Apart from vecna-blooded, most divinations have a casting time of longer than 1 round. If the wizard on that plane sets the time trait to 360:1, he can do in 1 round of real time what others do in 1 hour. And so on. So by the time you use metafaculty (or similar), it's already over. I think we could dismiss the time trait trick and the ultra-paranoid no-name Wizard will still be unassailable.
Now for some ideas on how to kill said Uber-Wizard: A Cleric with Initiate of Mystra could get through AMFs and dead magic zones. He could probably find a way to make the epic escape artist check (though does that apply to prismatic walls? otherwise, that strategy is useless). Then he'd still have to deal with the Wizard and some of his minions, though. It'd probably best to be mindblanked and invisible in that case.
Alternatively, wish can get one into the plane.
Transport travelers. A wish can lift one creature per caster level from anywhere on any plane and place those creatures anywhere else on any plane regardless of local conditions. An unwilling target gets a Will save to negate the effect, and spell resistance (if any) applies.
Emphasis mine. Even if that doesn't work, Greater Wish can do anything. If we allow Contingencies to be worded in a complex way that always works, Wish should get the same benefit. Even if
that doesn't work, you could always burn some XP and create a permanent portal to the Demiplane.
Not sure what my point was anymore...