Yes! He can transform back. Hes not rotted enough like the other Frostbrood but he’s undead like Nefarian when he came back in Blackwing descent! So he can still change and such and blends in as a Deathknight.
Personally im not sure about skeletal dragons shifting back to their humanoid forms..maybe they can maybe they cant? Its up to the rper!
I’d think it’d definitely depend on the dragon. I could see it depending on a lot of things, like how good they were at shapeshifting before they died and maybe how they were raised. My own frostwyrm is pretty skeletal (though not as much as the frostbrood drake mount) and it shows through when he shapeshifts, but he was pretty young when he died.