So does that make a restart (with <Option> key) a necessary final step following a Restore, or is there any alternative way of ejecting the backup drive/volume (without a restart) once the Restore has completed?

Thanks.

Right, you can not eject the disk the machine used to start. So, if you started from a volume on an external drive (and then copied/restored the external volume to a volume on an internal drive), you will have to reboot from a volume on the internal drive (or some other external drive) before the external disk will be ejectable.