The Question and Answer section for A Farewell to Arms is a great resource to ask questions, find answers, and discuss the novel.
Frederic Henry feels useless when he discovers everything continued to function in his absence.
Catherine explains her fear of the rain in the following way;
"'...sometimes I see me dead in it...And sometimes I see you dead in it.'"
I need to look further for the information about Frederic.
Yes, this novel is a "study in doom" because Hemingway has a sick twist for his characters. It appears that Heingway is intentioannly setting up his characters for a terrible ending.