If the rebels were pro-slavery it would make siding with the Imperials a much easier decision too.
Of what I know of nord history, there is no mention of slavery. They prefer to kill enemies, not enslave them. Hell, the nords didn't even take Aleyid slaves during the liberation of the humans of Cyrodiil in the 1st era, when the nords were super hardcoe, and willing to wage a war against the newly liberated Cyrodiils for continuing to worship the merrish pantheon.
If you ask me, I find it to go against nord culture, and find slavery to be a merrish institution. And because the nords are not very friendly with merrish ideas, I doubt this would be something they would pick up on. Again, they prefer killing their enemies, not keep them around.
ONLY in the Mages Guild. It is legal everywhere else, save for Morrowind and HIGHLY regulated in Summerset Ilse. Oblivion did a piss poor job, as it should be known that the government in Cyrodiil did sponsor necromancers, as they're a great group to dispose dead bodies of bandits. Same with High Rock. Nothing was really mentioned about Skyrim, other than bodies tend to be hidden in snow, bodies decay too quickly in Black Marsh, necromancers in Valenwood "become one with the forest" and the bosmer aren't fond of it because that's playing with food, the redguards tend to give the dead Arkay's Blessing to nearly every body (save for mass battles), Elsywr doesn't give a [censored], and Orsimar is unknown.