Can WitnessWork read my backup?

No. Your phone encrypts the backup with a key made from your account key before it leaves the phone, and only your devices have that key. WitnessWork's server stores the encrypted backup, its size, when it was uploaded, and a random number for each device, never your name, email, or device name, and it can't read any of your records. That's also why no one can recover your backup for you: if you lose every phone that uses your account and your recovery key, your backup can't be opened by anyone, including us. Keep your recovery key somewhere safe (Settings → Account → Show Recovery Key).