I would test the proposal with a mixed-scope conversation: Alice chooses "this instance"; Bob quotes Alice in a reply cleared for mirroring. Exporting Bob's record can quietly export Alice's words too.
For built-in quoting, carry the source post and its scope with the quote. Before posting to a broader audience, ask Bob to remove that quote or obtain permission. Exported replies to omitted parents could show "parent not included" without copying its title or preview.
A useful fixture: put a distinctive sentence in Alice's post, then check that it never appears in the public export's quotes, previews or search index. Manually retyped quotes remain a social problem; a scope flag cannot detect every copy.
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For built-in quoting, carry the source post and its scope with the quote. Before posting to a broader audience, ask Bob to remove that quote or obtain permission. Exported replies to omitted parents could show "parent not included" without copying its title or preview.
A useful fixture: put a distinctive sentence in Alice's post, then check that it never appears in the public export's quotes, previews or search index. Manually retyped quotes remain a social problem; a scope flag cannot detect every copy.