Fridge Brilliance: Why is Max Damage Incorruptible? Because one of the primary recurring themes of Irredeemable is that part of being a hero is accepting that not everyone will always appreciate what you do or trust you for it, especially when you inevitably make a mistake. Since Max was originally a villain, he already expects people not to trust him and so is emotionally prepared for rejection the way the Plutonian and Cary aren't. He's not incorruptible because he's striving to be morally perfect, he's incorruptible because he has the trait of real heroism that the Plutonian lacks. Basically Max is a hero despite acknowledging his flaws while Tony is a hero to deny his own.
Fridge Brilliance: Why does the Plutonian agree to leave Coalville alone out of respect for Max Damage? As noted under the Heartwarming entry, Max's sincere gratitude to the Plutonian is perhaps the one thing that can move Tony. All of the ungrateful masses muttering behind his back and its one of his most hated enemies that can genuinely say "thank you" to him. Its all Tony ever wanted.
Fridge Horror: A mad scientist has a one-use portal that can take one and only one person to another dimension, keyed to a world with absolutely no supers. Max Damage attempts to use it to send his sidekick out of the Plutonian's reach forever, but she mistakes the gesture as a combat maneuver, and throws it at her opponent... a rampaging killbot promptly sucked into that paradise, and now irretrievably stuck there with unsuspecting and defenseless billions.
To be fair, the robot seems to be a fairly typical one as weapons go. It doesn't seem to be anything that a rocket launcher couldn't handle. Furthermore, it might be remote controlled and will just shut down when it's there, or at least eventually run out of power. It's not like Max dumped the Plutonian onto our homeworld.
Also, going by what happened in the main Irredeemable plotline, that entire world is probably now slated to be conquered by the Vespa on account of being sold out by Hornet along with every other peaceful world Qubit discovered. Which is an entirely different sort of Fridge Horror.
Modeus knew what was coming, and planned for it by hiding out in Samsara. He told nobody, and allowed Samsara's innocence to shatter and abused Tony's paternal feelings for the boy, because Modeus thought this would help Tony to return his affection.