1) This has been requested before but not sure what the resulting discussion was (there's an entry for it as FS#9126 from when feature requests were still allowed on the tracker). Implementing it would be fairly trivial as long as you didn't expect a bookmark at, say, 0:03 in a track to rewind back into the previous track on resume. Edit: No, wait, it's not trivial, bookmarks are more complicated than I thought. It's still vaguely feasible but it would take some investigation to really work out what the restrictions might be.
2) You can do this already. Set "Bookmark on stop" to "Yes - recent only" and turn on "Maintain a list of recently used bookmarks". If you manually create a bookmark it will get written to a .bmark file next to the mp3 and also to the recent bookmark list, but when they are automatically created they will only be put in the recent bookmark list.
3) This is a bug, FS#2720 - idle or sleep poweroff does not trigger creating a bookmark. There is a patch there but it's very old and may no longer apply to the current code. You can still hit "resume playback" to get back to the point you were at, if you do it immediately, but this will be forgotten if you play anything else first, of course.