I think you're getting hung up on the word "symbolism" tbh. And taking it to mean that the author intended this thing to be representative of this thing. When like some people said, really, your specific example of the Holden on the hill is about establishing tone, mood, and characterising Holden.

It's not so much about it symbolising that he's isolated from his peers so much as establishing that he is.

Anyways, the best thing my lit teacher ever told me is that there are no right or wrong answers. Just use the text to bring evidence to support your interpretation.

At the end of the day your argument has to have supporting evidence from the text and it has to make sense.