Seems Microsoft Support was too quick to blame pirate software for my Outlook problems. After four calls totalling at least six hours case number 1101 *** *** is finally closed. And the culprit? Too many emails in a single folder.
But it's certain that the fact I had a Chinese version of Office on my computer was the cause of the first problem - Chinese language folder names. Once that was fixed, Microsoft leapt to the conclusion that because it was pirated then the next problem (emails received after 27 July 2008 wouldn't import) was due to the same fact.
In actual fact, I would probably have had both problems even if the Office software had not been a pirated edition.
To fix the second problem we split past emails into 'by-year' folders and imported each year separately. Thus Outlook does not tolerate too many emails in one folder. Splitting them into years meant we stayed under the maximum per-folder limit, and the import was successful.
I only decided to get Outlook because my previous email client - Windows Mail - is not full-featured. As I noted in the earlier blog post about this issue it did not allow me to control the appearance of my signature on reply emails and forward emails.
Now I have total control.