What is the significance of Holden's absence at Allie’s funeral several years prior, and how does this particular circumstance relate to Holden's characterization through the whole book?
Holden was hospitalized because of his case of pneumonia; per what we see of Holden, he is a chronically sick individual who cannot stay healthy.
Holden was forbidden to go to the funeral because he was at fault for Allie's death; this is why he continuously thinks about Allie throughout the entirety of the novel.
Holden was still hospitalized and unable to attend because he had a self-inflicted injury to his hand; throughout the novel, Holden continues to discuss the possibility of suicide.
Holden does not believe that people should be buried just so others can come put flowers on their graves, so he didn't go to Aliie's funeral as a sort of protest, furthering his development as a rebel.
Holden was still away at school, unwilling to take leave from his academics as he was worried that he would fall behind in his studies.