The WeiDU readme lists the language names that HANDLE_CHARSETS handles by defaut. Those names are in the list, so it should associate the right character encoding for them. Providing the table is only necessary if an unlisted language is concerned or if a mod uses an unusual language name for one language.
Provided the iconv binary for Windows is located in %MOD_FOLDER%/iconv directory (%tra_path%/iconv is the default location if you don't specify iconv_path, as written in the readme), the second code should work.
So, compared to v9, you need to move iconv.exe and the dll in an iconv subdirectory.
If you don't move iconv.exe from its location in v9, you need to add iconv_path = EVAL ~%MOD_FOLDER%~
Thank you for taking the time to fix this mistake I made years ago. I made a similar change locally but never posted it and probably never tested it either.