Jack Malloy is my favorite character in the book. He is his own man and extremely intelligent but in the military environment, he just doesn't fit in. Ironically, many of the characters in the book who were perceived by the officers as "screw-ups" would have made the best soldiers in wartime. This is something that SSgt Warden seemed to realize. His relationship with Prewitt is interesting. One one hand, Warden respected Prewitt as a potentially great soldier but on the other hand, hated him for who he was and happily went along with Captain Holmes efforts to break him.
During my time in Marine boot camp, I served briefly on guard duty for the brig (they called it "Correctional Platoon"). The conditions there are much as described in the book. Those men go through hell. I remember how they had to wake up and completely disassemble their "racks" (bunk beds) and stack the rails up against the wall. At the end of the workday (usually digging ditches or doing other scut work), they would have to reassemble them. Sometimes the Sgt. on duty would take a handful of bolts and throw them across the room after they left for the day, adding to the confusion that evening when the poor guys had to put their racks back together again.
I am sure Mallow was based on a real person (as most of Jones' characters are). There was just too much detail about the IWW (Wobblies) for Jones not to know such a person. Also since Jones was only in his 20s when he wrote FHTE I don't think he would have known enough about the Wobblies on his own to completely invent such a character. Most likely, Jones did meet an ex-Wobbly much like Jack Malloy. I would also like to know which real-life person Jones based Sgt. Warden on.
BTW, Robert E. Lee Prewitt was based on an Army buddy of Jones' named Robert Stewart.