evidence, and amazon's response, suggests hardware failure - device stopped being able to write information to its memory
that is NOT caused by your activities, though I get how it can seem that way.
one way to put your mind at rest could be to perform all the same activities on the replacement kindle, when it arrives [ you have nothing to lose because if u do manage to brick it, you can send it straight back And say to me nah nah told you so... "
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the worst you should be able to do by feeding it poor quality data is to confuse the indexing process, but a factory reset would have fixed that.