Why did Keisuke survive being cured from Line by Akira's blood while all others affected died?
He was part of the same experiment to create Line itself, and it's quite possible he has acquired an immunity to some of its effects. This also explains Keisuke's high, abnormal compatibility with Line, and how its positive effects were retained after contact with Akira's blood.
Akira suffers from some Character Derailment and Badass Decay throughout the course of the novel, being slowly unable to participate and win in fights, degrading more than simple malnourishment would. He also becomes less decisive, shown in his loss in ability to analyse an enemy's fighting movements and struggling relations with other characters, and has to depend on others. Then, remember Akira has suffered considerably emotionally, having been thrown into Igura without warning, having friends killed left and right, random killers running after him and an inexplicable mystery in Toshima.