You would be better off if you restored the power slider functions to what they were. Then you could sleep in Koreader. That should be all you need to do most of the time, and when you really want to poweroff, the KSM poweroff function is just a couple of pokes away.
The bookmark does not set where the book will open. It is there so you can return to that page using a list of bookmarked pages. There is a "History" menu item that will have your current book listed at the top. It is in the File Manager menu, as well as the menu you see when reading a book.
Luck;
Ken
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