It is not definite that you will lose root, you may lose it, depending upon the way the update script is written. Some update scripts are written to delete files app by app, some delete files recursively, and some do a combination. If all the root-related stuff is not deleted by the update script, you might actually retain root. In addition, some root methods now also include a method to retain root even with updates.
2) A TB backup is only usable when you already have an operating system in place. A TB backup is not the same thing as a Nandroid backup, which also backs up all of the working partitions on the phone, so that in the event of the phone not booting, you can boot into your recovery and restore the Nandroid backup and boot your phone again. With TB you can only do this from a running system. Of course, the Nandroid backups can only be made from a custom recovery, and will vary from phone to phone. In the D1 it backed up just about everything, including the boot.img, etc. In the BIONIC, with our locked bootloaders, custom recoveries are only backing up /system, /data, & /cache partitions.