Exiv2

CRW files are in CIFF format. CIFF format consists of a header and a hierarchy of directories and entries, similar to TIFF (but easier to deal with). Some of the CIFF metadata can be mapped to Exif metadata.
The initial scope of the support for Canon CRW files is to read CRW files, parse the CIFF structure and map the known and suitable tags to Exif metadata. CIFF tags for which there is no mapping defined will be ignored. There will not be a new CRW or CIFF type of metadata.
Writing to CRW files will perform a reverse mapping from Exif metadata to the CIFF tags found in CRW files. E.g., it will be possible to adjust the date when the image was taken by reading the CRW file, modifying Exif.Photo.DateTimeOriginal and writing the Exif data back to a CRW file. Exif tags for which there is no mapping defined will be ignored.

Canon THM files are generated together with CRW files. They are Jpeg files containing a 120x160 pixel thumbnail version of the image and the complete Exif metadata for the image. However, this Exif metadata does not contain any preview or thumbnail images.
Since Exiv2 determines the file type based on its contents rather than the file extension, THM files are already fully supported.

Should the library or utility transform the THM image to an Exif thumbnail when reading Exif data from a THM file? If so, what about writing to THM?

Write support completed.