So for Mitarai, I see six possibilities.
1. Mitarai becomes a Remnant of Despair, but reforms on his own due to his powerful desire for hope. This ties into why he is so distraught by how dissimilar he is to Naegi.
2. Mitarai becomes a Remnant of Despair, and does not reform - he is the mastermind.
3. Mitarai becomes a Remnant of Despair, and does not reform - he is not the mastermind.
4. Mitarai becomes a Remnant of Despair, and does not reform - but is trying to. He is not the mastermind, but is involved.
5. Mitarai becomes a Remnant of Despair, and does not reform - but is trying to. He is the mastermind.
6. Mitarai - and this is the "it's an anime" argument - becomes a Remnant of Despair, and is made by Junko to create an anime that fills people with despair. All the while however, he is struggling to defeat the despair.
My guess is it's 1, 4, 5, or 6. I just don't see him still being a Remnant of Despair except for the 4th and 6th scenarios. Ultimately, I am leaning on 1, 4, or 5, just because I feel that all of the extra stuff with Monaca and Fukawa and Hagakure and Hajime and Byakuya and Komaru just distracts from the "it's an anime" scenario.
Then again, the "it's an anime" scenario is more believable if we operate on the assumption that Mitarai is adapting the events of real life into anime. The only problem is that we then have to forget that Mitarai is drawing things that take place ahead of where he is. The only explanation that I can conceive is that there is some timeline shenanigans going on with those scenes.
EDIT: Also, is it fair to assume that Izuru Kamakura is the most powerful character in Danganronpa? We see that he - without even trying - knocks Mukuro into a wall so hard that she actually cannot walk right away. Shit, if Izuru wanted to, he could have given her a nasty scar. I guess it stands to reason - he is the Ultimate Soldier, Martial Artist, Gymnast, Swordsman, and Doujinshi Artist.
And speaking of Izuru, what kind of luck does he have? I'm aware of about four different people with luck - Makoto, Nagito, the girl who won the lottery at first but lost her ticket (thus allowing Makoto inside), and Izuru. The first three, interestingly enough, have a form of bad luck on their side. Makoto's door frame is busted? Well, that solves the first case! Makoto chosen to be executed? Well, he survives and goes onto solidify the bond between everyone! Girl loses her ticket? Well, she avoids the killing game (though I'm curious to see if they ever tell us what became of her). And of course, Nagito needs no explanation. Izuru though, is he bad luck like everyone else? Has Danganronpa ever expounded upon characters with good luck?