no, it's an app trick. not all apps allow it. it's on the roadmap to be integrated in lemmy itself btw.
if the instance someone is using is defederated from that instance, would your profile appear empty?
there is an entire instance dedicated to that, use the lammy explorer to find stuff. be aware that your instace could have defederated from that instance.
no idea, you should search issues on github.
Anyway comments are replicated across all instances, so nobody stops someone from running a modified instance that does not abide to the comment deletion request. Same for posts, idk about images.
you should consider everything that get published public forever.
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