Just to complete this thread, in case someone else experiences this: My problem was solved (the built-in Oxford dictionary reappeared in the book list, and consequently I was able to see the "Change Default Dictionary" option again) when I restarted my Kindle. I'm not talking about turning it off (holding the slider for 15 seconds) and turning it back on -- that didn't help -- I'm referring to the "Restart" option in the Settings Menu.
Still an odd quirk (wonder why it happened), but as long as it solved itself I'm not too worried.
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