Hybrid back-up and disk replacement Please find the answers below:
1. The hybrid backup is designed to run local backup at first then to cloud. If your local storage device (USB drive in your example) is corrupted or unavailable, the hybrid won't run a cloud backup.
2. The drive replacing scenario will work - the only thing you should do before continuing your hybrid backup is to change your existing File Storage account that linked to the USB drive to a new one and specify the same backup prefix for it (see "Advanced settings"). Once it is done, your previous local backups will be synced and you can continue with the hybrid backup. You can find the main idea of replacing & sync
here.
3. Your calculations are correct. It sounds reasonable to have at least 2 full backups.
Please let us know if you have any comments/questions.