You can't delete the notes from history, but you can revert to an earlier state of the note. This is to make it harder to vandalize notes, as anybody who wants to get rid of them (or even make bad edits) has to manually go through every single one. A note that's been deleted, as explained above, is still marked on the history so that it can be fixed more easily.

As for sets, I'm not sure what your problem is. Could you explain further? If you're just asking for clarification on the terms, a maintainer is somebody who doesn't own the set, but is qualified to edit it and add more images. An owner is somebody who created the set, and has the authorization to add more maintainers. If you're wondering why you can't add posts to somebody else's set, that's because you're not a maintainer of it.