The attached patch issue9291.7.patch (which is essentially an amalgam of 9291.patch & 9291a.patch with some tweaks of my own) does appear to solve the issue. My Windows setup is UK, so if any of the people still watching this issue could test against a non-English Windows, that would be useful.

Even this fix does leave some room for encoding mismatches between the stored values (mbcs encoded) and any string passed to guess_type. But it's not clear how that should be handled, and at least it doesn't crash out on .init.