The Kill Order question

It doesn't say he dies, but he would have died from the Flare.

I know he was sick but I think it was in the end when everything went to hell in a hand basket

Yes Mark does die if I remember correctly. It was inevitable for Alec, Mark, and Trina to die. They had all caught the Flare and they started to show obvious symptoms toward the end of the book. “Let's get this started, people. It won't be long before we all lose our minds.” Either way they tried to do the most they could before they died. They find the flat trans and get Deedee out but Alec crashes the Berg into the flat trans, killing Mark and Trina.