I won the war for Ulfric and he gave me permission to kill them but do I really have impunity in this? Can I go around to Imperial Camps now and smash away to my hearts content? I ask because their commander will always survive the attack so that leaves a witness.

"Essential"

So if you're playing on console, no.

Can't kill the Camp Commander, but all the other ones are fine.

I find Storm Call from a hidden position to be the way to do this.

You can, but for some reason, the Commanders are essential. Same for the Stormcloaks assuming you do the Legion. All except their commander can be killed.

Perhaps Beth planned a DLC that involved the CW.

No, because the Legate in charge is essential throughout the whole game. This is probably a bug that Bethesda hasn't addressed.
EDIT: you can kill the not so high ranked ones though.

Can you legally kill the normal citizen imperials just living in towns? Like camilla valerias?

I know you can't, just think it would be funny if you could.