Since spirits can die this second death, they are not immortal to God, but as a matter of fact they are immortal to us. They can be restrained, but can't be killed. What is impossible to man is possible to God, that's what I meant.
I'm sure they're not, those are divine beings, aka angels, hosts of heaven. They don't die - UNITL God cast them into the Lake of Fire to die. These rebels will be judged. There's no place for them in New Jerusalem. If there were mortals, why would they die "like" men? Why not just die as men? And how come are they able to stand face to face before God (Job 1:6), since no man can see God's face and live (Ex. 33:20)?
And the angels who did not keep their proper domain, but left their own abode, He has reserved in everlasting chains under darkness for the judgment of the great day. (Jude 1:6)