That’s because keys are supposed to be unique! So adding the post id to the avatar is correct in this case. I might want to raise an error in development mode if two elements use the same keys. If you want to commit the post avatar key to master that would be fine.
That’s exactly what it is. By default I think a widget is supposed to re-render its parents, but the shadowTree is a performance optimization that avoids too much re-rendering and requires you to be more explicit.