Camera app does it.
replyNope; the lockscreen camera is actually an entirely different app, or not-quite-app thing. (Note what happens when you open the camera roll in the lockscreen camera.)
replyIf the app doesn't have any sensitive information, then yes. We already do this with home screen widgets today, so clearly there's no real barrier.
replyApps can have vulnerabilities (intentional or not). It's a contrived example, but maybe some weather app has a custom icon feature, which means you can then browse photos by opening it on the lock screen and going to that setting.
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