What touches of characterization in the early part of the story prepare us for Paul's eventual suicide?no
In Cather's "Paul's Case," Paul is isolated from the beginning. I suggest that what's at the center of Paul's misery is his disdain for everyone and everything around him. Which came first, the depression or the disdain, I don't know, but the only environment and people he accepts in Pittsburgh is the theater and the actors that perform there.
"Disdain" may not be a word that's accurate at all times for what Paul feels. Sometimes he just seems to feel boredom with the tediousness of things, the sameness of everything: his father waiting for him at the stairs, for instance. But disdain covers much of his attitude toward others: his teachers, most notably.
He also intuitively feels he knows something no one else does, and he does: suffering and isolation. Unfortunately, that's not what he thinks he knows. He thinks he knows more about the finer things in life than others do, and that makes him "better," more aware than them. Thus, he will never accept them.
Once the theater is made off-limits to Paul, he is doomed unless he can escape Pittsburgh. When it becomes clear that he will not be able to stay in New York and live life like he needs to, he has no option except suicide.
I suggest Paul's suicide is absolutely motivated by everything we know about Paul, not only in the beginning of the story, but in the entire story. The suicide is the natural ending to the plot. It's the only possible ending for Paul's character in early 20th-century America.
The story is an accurate portrayal of a person suffering from a "case" of depression.