>rotate and flip them losslessly in ThumbPlus
Yes, but then I have no protocol or log file about the action I made. The way 1. click on Batch to make protocol, 2. rotate and 3. batch again for protocol seems to be longer then rotating and flipping (!) with exiftool. But I try that. Now with the batch, it takes about 10 sec for 1 image (old slow PC).
>rotate and flip just their thumbnail in the database
Thats a very good hint. But for the endversion, I think it would be better to place an save the orientations in Exif-Fields than in the database. The scans are for an archive, and maybe in 100 Years, jpg lives but Thumbs-DB not :-(
Possible is also to make a protocol about the diffrence between Exif- and Thumbs-Orientation.
When a thumbs ist rotated, it seems that this information is saved in the database as a new file type. In the table "Thumbnail", all jpgs have normal the number 8 (in my sytem) as filetype. When I rotate the thumb, the Filetype change to 56 (for 180° Rotation) or 24 for 90°. Maybe I can save the original orientation from Exif in a user-field and compare it with the Filetype. That result I could export to a txt-file, that maybe in 500 years can be read :-)
Or I export the result to exiftools and let the PC set the new orientations in exif-field from the database over 1-2 nights.