I smugly assumed that I'd finished learning from my mistakes for the moment, but there were a few more pitfalls ahead than I'd anticipated. As before, they're small technical details because I'm too close to it all to be able to stand back at the moment. For what it's worth, here's what I've learnt:

1. No matter what they say, Kindle Direct Publishing isn't about just clicking a button and you're published. It takes a few hours for an e-book to appear online and a few days for a paperback to feed through. It then takes up to a week for the two to be linked together so that you can click between the two without doing a new search.
Lesson: When I launch my next book, I'll publish first and launch a week later, just to be sure everything is where I want it to be.
2.  A book's enrolment in Kindle Select, which is necessary to be included in Kindle Unlimited and to schedule a promotional offer, doesn't start until the book is published. Once the Kindle Select period (three months renewable) starts, it's possible to schedule a promotion. The promotion has to be scheduled the day before it starts. That means that I can't make my book free on the day of publication, only the day after.
Lesson: When I launch my next book, I'll publish the paperback version first, then the e-book, then schedule a promotion in time to launch a week later, sure that everything is where I want it to be.

3. For reasons that I don't fully understand, although the e-book description came out fine, paginated as I wanted, the pagination was lost in the paperback description. Eventually I fixed this with html tags: <p> at the beginning and </p> at the end of each paragraph. 
Lesson: Put the html tags in to start with.

4. Once everything is uploaded for publication, it's impossible to apply myself to anything else and it's also impossible to relax. This seems to be a good time for scheduling tweets, and posts on Facebook and Instagram and writing blogposts. Whether it's a good idea to do this in advance remains to be seen and I expect there's a limit beyond which self-promotion becomes counter-productive.
Lesson: If I do the sensible thing and publish a week before launch, I'll magically create a window for promoting the book.

5. The launch event I set up on Facebook does seem to be a useful way of keeping in touch with potential readers, though I'm not seeing my own messages on my timeline or in messenger, so I don't actually know where they appear. It'll be interesting to see how many of the people who signed up to the event end up downloading a free copy.
Lesson: This seems to be worth doing again, though I need to provide clearer instructions.
6. Messaging people individually generates a significantly higher level of response than grouping people together in the 'to' box.
Lesson: It's worth the extra clicks on Facebook. People either ignore messages on LinkedIn or look at them less often. It might just be a slower burn.
7. The limbo period between upload and publication is also a good time for updating my website and making sure all the links are working.
Lesson: They weren't, so it's worth checking once in a while.

8. I figured out that I just need to put a link to my blog to have the right image showing on Facebook, which saves me piddling about with screen-shotting and clipping and uploading. If I include an image caption, though, it ends up being incorporated into the text nonsensically.
Lesson: Leave out the image captions (it's ok, they're all free to reuse without attribution anyway, I was only being polite)

9. I've been uploading promos on various Facebook book promo groups and it's amazing how many other authors are busily doing the same thing on a daily basis. Where groups have so many posts that I've been bumped too far down the timeline to be found, I've re-posted, but tried to make it slightly different to catch the eye. Where there's less traffic, I've updated the existing post by adding a comment. I've had some feedback from other authors, including the useful advice to provide the link to amazon.com as well as co.uk.
Lesson: I'm not sure, because I don't even know if this form of promotion works. It may just be indie authors posting to one another.