Re: WWYCD: Mirror Universe
Red Death is a villain, and a very nasty one at that, who has made it impossible for other powered versions of himself to exist in the multiverse.
He would probably go on a typical rampage, see if anyone can stop him or raise the dead in this universe, a lot depends on if this is the mirror of his origanal timeline or his current one, and if this is before or after his death.
He wouldn't care about much right now with Invincible dead and Jane having gotten him to kill himself, coming back from that would screw him up more than normal.
Arthur Kent hunts down and kills his alternate self, he has killed his evil self before, and he will try to gather the local heroes to come back to his world for the coming war with Death.
It is possible his father will notice the dimensional changes caused by this and help him get home, but he may miss him with the other such events going on right now.
Terry tries to find a way back home, it depends on if the other X-Men (or any X-Adults) have come with him, he probably ends up dead if he doesn't get back to his own world.