OK, I've factory reset the KP and re-entered my data. The device downloaded a couple of titles I selected from the Cloud, and three I bought afterwards. So far, so good.
I agree, if the KP loses track of consensus reality again, I'll contact my retailer (readership.com.au). Prayer and electricity mightn't be enough.
Thanks very much for your prompt and informed help. I'm new to Kindle OS (such as it is), and appreciate you sharing your experience.