Hello, Dave!
I want to report the current status: I have first tried an erase-then-copy on the encrypted backup-drive. After 12 hours the backup was ready.
Restarting from the encrypted backup-drive was however not possible.
Restarting using the Startup Disk preference pane put the mac in a waiting loop: nothing happened after the white/grey screen!
Restarting while holding down the Option key shows the following bootable drives: the internal drive, EFI-Boot and the TimeMachine-drive.
Choosing the EFI-Boot shows Startup screen and the two user described below (a Guest-account and another one labeled "[updated needed]").
I will now delete the Backup-drive and partition it as (simple, not encrypted) Mac OS X (Journaled). After this I will start again an erase-then-copy, try to restart from it and report the status again.
Until then,
Vlad
P.S. I am still very suspicious about the firmware-password. Right now I am not able to restart in the recovery partition (see also my question in an
Apple-discussions-forum), I am not able to reset the NVRAM and I am not able to make a safe reboot (due to FileVault2). I don't know if all this has something to do with my question here, but it address at least the same strange behavior after reboot.