Seeing as the main goal is to "just use it for reading" (a goal I am sure is the main importance to all of us), I know it's not the answer, but, if it seems that never powering off is the workaround, why not use that, and live with the fact that the power off mode might never get fixed. If you can get a month of life using sleep only, then why not?
Make sure you report it to Kobo though, so they have a fighting chance of tackling the problem.
Matt