Looking at the problem more closely, this is an Excel bug.
Your proposal of converting the utf8 to big5 doesn't work, at least with my version of Excel 2008. It fails to understand the text at all. No option is provided to select a character set.
Converting the utf8 to utf16-le works sort of. It causes excel to drop into the import wizard, where you have to specify that this is seperator delimited, and the seperator is a comma, despite it being a file with "csv" extension. It will further prompt you to interpret some of the fields, which might be useful. This means you get, even with utf16 around an 5-6 step process to load up the files. It's hard to know if that meets your use case, but the limitation is strictly on Excel. The text does appear to be intact at that point, however.
Customers using other versions of Excel will see different behavior. They might already have success with the above proposed change.
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