Sarah Canary Q2 - '“The dress sheds bullets,” Harold told him.'
There are several recurring motifs in the novel that are with us from beginning to end. The one-winged bird, mirrors, and Sarah Canary's dress are the three that stood out most for me. Are there any others? What do you think they symbolize?

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The stand-out one for me was what I mentioned in the Q1 thread, that everyone around Sarah projected their own presuppositions onto her, and they were almost certainly all wrong. I don't mind that she herself remains a mystery, though the novel wold have been improved for me if there had been some clues as to who she thought she was.
The world isn't rational. Harold (focusing on the dress) is maliciously insane though it is mentioned as being an unusual dress where the buttons are decorative rather than undoing. Chin's one winged bird is a symbol of something useless - but Chin is someone looking for symbolism and meaning.
The symbols and meanings of them are what the characters invent for themselves, building on what @RichardAbbott said.
My take on the one wing
The bird with one wing - it seems obvious that this is a reference to something not being whole. I'm not sure I'd say 'useless'. Sarah herself is likened to a bird quite frequently, and it's right there in her name. She sings like a bird. She behaves like a bird. She's not quite whole, either. The other characters are not quite whole, either, so this seems to reference all of them. There are some interesting quotes on this subject:
Sarah shows us that one one chopstick can be extremely useful, in the right context. And speaking of context:
This, I feel, is a reference to Sarah Canary's otherness. She's an alien, or she's an angel (and maybe an angel with one wing?). In any case, in a world that supported her, she might be quite something. This same argument could be applied to immigrants, women, or handicapped people.
Toward the end of the story, the one-wing motif comes back again, this time when Harold compares Sarah Canary to a one-winged butterly.
I'm not quite sure how to reconcile the message about helping causing harm with the rest, though.
At the very end of the novel, we get this, another reference to breaking. But in this case, it's the thing that is broken that has become the butterfly, whole and presumably beautiful.:
What is broken can be whole if you change the context. Is a broken heart a chrysalis?
One last reference to a mirror came in the story of Caspar, the prince of Austria, who was hidden under a stair. Later he was killed in a park, and a note was found beside his body - the note was written backward so one needed a mirror to decode it. Is this another hint that we need to really see ourselves in order to change?
Mirrors are also a big motif. As @RichardAbbott pointed out, each person sees something of themself in Sarah Canary, so in a way she is a mirror, herself. It's also mentioned more than once that 'the wild woman cannot see herself in a mirror'. I'm wondering if this means that somehow women struggle to recognize themselves, or somehow fail to see their own qualities?
Late in the book, Chin muses to himself:
So stories are also mirrors, in a way. Chin goes on to think:
This seems to circle back to the idea that Sarah Canary is a mirror - we see something of ourselves in her - or at least the characters in the novel do.
A for the dress, Harold seems to think it's some sort of armour:
The dress is mentioned very often, and it's usually how they can pick Sarah Canary out of a crowd. It never seems to be wrinkled, or get torn. It's so tight about the next that Adelaide wonders how Sarah can breathe. Is it just a dress? Is it also a costume? Is it armour? Is it an environmental suit? Is it symbolic of something? I'm not sure.
Sorry for posting four comments in a row - I just wanted to explore each of these in its own post.
One more motif that just occurred to me - B.J. has frequent conversations with inanimate objects. Is that because he's more comfortable with them, or is there some deeper meaning?
BTW (here is as good a place as any) I really like your technique of using quotes from the book as the discussion seeds