Unless there is an undocumented change in MC15, a restore always resets the cached thumbnails and new thumbnails are created from the scratch by reading the source cover art, video and image files.
You can replace the files in the thumbnail cache with manually backed up files after the library is restored (and when MC is not running), but when MC is restarted it may detect that the thumbnail cache is not exactly synced and the cache will be erased and recreated.
The only sure way to back up the library data and the cached thumbnails is to close MC and manually back up the library files and the files in the thumbnail cache (in addition to storing a library backup file if the intention is to back up & restore also the user settings). Then the thumbnail cache will be exactly synced because the library has not been run after the thumbnail files were last saved.
The restore process would then go as follows:
1. Restore the library (if you want to restore the user settings).
2. Close MC.
3. Replace the library files and the files in thumbnail cache with the separately backed up files.
4 Start MC and have everything nicely restored.
BTW, 1.7 GB is quite a big cache size. How many files of each media type do you have? I guess that if you have a huge amount of image files the cache size can grow significantly. My cache appears to be 544 MB for my main library (about 80,000 audio, 1,000 video, and 11,000 image files).