Well, I think I've managed to answer my own question. From further googling, I suspect the problem was Evolution-related, although I can't see how "import" can screw things up in the application (KMail) that's being imported from.
Anyway, it turns out that those extra files are not required and can be safely dispensed with, so that's what I've done. KMail runs OK and looks just the same as it always did. I might have another go some day with Evolution but I think I'll leave well alone for the time being.
Although the root cause has not been found, I suppose I'd better mark this thread as "solved". Kicking myself for not having backed up its mailbox in the first place, as I didn't think importing from it could cause any damage.