tl;dr, my oppinion: dilemma - no power, no devour, but someone goes insane, first player's choice; bargain - no power, no effect
Regarding prisoner's dilemma, I would rule it as a tie, so players decide what happens (like when you start a new character mid-game and you can choose to fail or pass your your personal story, because both criteria are met). Which most likely means someone goes insane (or the least satisfied character may choose to start anew). In fact, no power should mean not getting devoured. Insanity however, is always welcome in Arkham.
Bargain of the gate is a bit trickier. It's a huge benefit usually, you're getting a free ride to an other world of your choice and you can even close/seal a turn earlier than normal (reckoning happens in mythos phase, you don't get delayed, thus you can move to the second area next turn). Or abuse it with find gate, but that would be heresy! It's a benefit, so you should have at least some power (= taking a risk) to be able to utilize it. Lurker works that way.