Well, that's what the Divination-fooling spell is for; so that I can make Dispater believe that he's in a Mordenkainen's Magnificent Mansion on the Material Plane while he is, in fact, on one on Dis.
A Planar Bubble should do the trick of making <that room> not count as Dis, correct?
Or I can just Contingency him, with the Contingency being activated as soon as he spends 1 minute on Dis, with
Painless Death triggering then.
It still has some knots (like getting the most paranoid being in the universe to allow me to cast a spell on him
)