I think you only have to RESET the kindle, to remove all traces of the jailbreak. You don't need a software upgrade; that's only needed when you install the "factory software" needed to do the jailbreak. You then immediately install a "user level" software, so any of those is safe to send back to amazon. But you aren't doing that now; just resetting.
So: just the RESET. That *will* remove the jailbreak, and since you are at software 5.8.9, you can no longer install a jailbreak after that. But, if you wind up returning it, that won't matter anyway. If you don't wind up returning ig, but it fixes your problem .... well, no jailbreak, but no line across the screen.
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