There's no way to know just from 'Henry Henry novel' whether he is the protagonist. It could be that he is just a minor character or part of a sub - plot.
I'm not sure which specific 'Henry Henry' in a novel you are referring to. There could be many characters named Henry in various novels. Maybe it's a relatively unknown or self - published work. Without more context, it's difficult to determine who this 'Henry Henry' is.
Henry in the 'Haunted by Henry' true story could be a historical figure. He could have been a soldier who died in a battle near a certain place and now haunts it. Or he could be a local hermit who had a strange reputation during his life and after his death, people started experiencing strange things and attributed them to his haunting. Without more details, it's difficult to determine exactly who Henry is, but he is clearly the central figure in this supposed true haunting story.
In 'the john henry story', John Henry was a heroic figure. He was a railroad worker who was known for his amazing physical strength and his unyielding spirit. He became a symbol of human resistance against the rise of machinery in the workplace.
Since I'm not well - versed in J J Fletcher's specific war novels, but in general, war novels might have detailed descriptions of military strategies. Maybe J J Fletcher's works also include such aspects.
I don't know exactly who J Andrews is in this story without reading it. But it's likely the protagonist or someone central to the events of the daydream - filled day.