If you activate WhatsApp (verifying your phone number) while having a /sdcard/WhatsApp/Databases/msgstore.db.crypt12 file present AND have a WhatsApp backup on Google Drive, WhatsApp will claim to restore your Google Drive backup (with your consent), but it only restores the media files from Google Drive but at no occassion the msgstore.db.crypt12. It won't merge your local backup and the cloud backup databases!
With local WhatsApp media files it will only offer to restore the local backup at activation but not the one from Google Drive, hence the need for an empty /sdcard/WhatsApp/Media/ folder. But with a local chat backup, the Google Drive backup restore process just imports media only anyways.
Ways to get to the key file to decrypt the *.crypt12 DBs:
- either access
/data/data/com.whatsapp/files/keywith root permissions - OR
adb backup com.whatsappand extract the key from the.abfile
I have extracted the key and found out that it's the same for all my devices (I always tested with the same phone number). For the decryption and restoring of the locally saved msgstore.db.crypt12 (like described above), it is also automatically saved on WhatsApp servers.
According to what i found out whatsapp doesn't allow backups in its manifest, so you would need to decompile and recompile the app.