Thanks for your thoughts everyone. Sadly, turning the Kobo off and on again seems to actually cause more database corruption, although it does help the Kobo recognise books that it hadn't recognised up until now.
I am (once again) trying a full factory reset, and I'll keep the firmware at 4.30 for now, as others seem to have had database corruptions with 4.31. I am leaning towards it being a hardware issue at the moment though, because of the very strange symptoms.
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