> Do you think that some of the filter data is stored elsewhere in "the database".
No, not to my knowledge.
> The release notes mention a TDO file. Does Thumbs9 use the *.TDO file?
No, TDO files were abandoned with v8.
> Do you know what global parameters, (e.g. "NameMask"), in the Thumbs9.ini file, MUST be changed in order to control the choice of selected file filter at start-up?
To me it seems that TP, at startup, is always applying the last-used filename mask, and it is stored in the FileMask= entry in the [FileList] section of Thumbs9.ini.