This worked for me for the "This account already exists on your device" issue:
After 2 days of trial and error, I was finally able to restore my chat history. Here's what I did:
- Install WhatsApp using Google Play Store but don't start the setup.
- Navigate to Settings > Passwords and Accounts > Delete the Google account associated with the google drive where your WhatsApp backup is stored.
- Grant all permissions to Google Play Services (reverse this after installing WhatsApp).
- Complete WhatsApp setup > enter Google account info when prompted
- WhatsApp will then prompt you to restore from backup. (enter your encryption key if you encrypted your back up)
- Revoke all permissions granted to Google Play Services in step 3.
All chat history should be restored
Worked for me... hope it helps someone recover their history