> Is there any other way to convert to SQlite?
I've never tried, but I read about a freeware tool called 'SQLite2009 Pro Enterprise Manager' (
http://sqlite2009pro.azurewebsites.net) that is said to offer import from MDB into SQLite databases. It might be worth a shot (and I'd be curious to hear about your findings).
> Alternatively, is there a way to check whether in fact the database reflects only file data?
Not directly, but maybe you could make a new database, configure TP to copy any embedded keywords from your images into its database (Options | Preferences | Metadata) when it makes thumbnails, and then compare the results to what you currently have in your real database.
One way to get a list of filenames and their keywords (and I'm afraid much more) is to Export the database to a text file using Database | Advanced | Export (don't include the actual thumbnail data in the export if you decide to do this, or you'd get a monstrously large file).
> As a desperation move I suppose I could export a complete set of keyword / image file pairs, rebuild the database, and then add back all those pairs (which would be a waste of time for the ones already found in the image files, but
would fill in the missing ones. I suspect it would take a while, too.
Theoratically that would be a good solution and you don't have to include thumbnail data in an export/import; ThumbsPlus can re-generate those if the files are available. That would speed things up. However, the export/import functions in ThumbsPlus are quite unreliable due to file format confusion and they tend to stumble over the smallest things (like multi-line annotations). Still, it could be worth trying.
> Is there a free DB tool, or a Perl or Python library, or something, that will read the .tpdb8 file format (I think that's a version of Access, but whatever it really is)?
They're regular .MDB files. Try the excellent and free [link=http://www.alexnolan.net/software/mdb_viewer_plus.htm]MDB Viewer Plus[/link] by Alex Nolan. Type *.tpdb8
in its File Open dialog to find and open your ThumbsPlus database files (by default it only shows .mdb files).