Mist Form is for one of the strains in Valenwood.
I think two things inhibit lore correct Volkihar: No underwater combat (so you can't fight them if/when they pull you under water) and I don't think you can create new animations, meaning you can't program them to jump out of the ice or anything like that.
A couple things:
1) Spriggans spawn from trees. You may hear buzzing if you approach a tree but you don't see anything. Sure enough if you get close enough to it, the spriggan walks DIRECTLY out of the tree.
2) Tunnelers from Lonesome Road come up from out of the ground. If New Vegas could do it, so can Skyrim, as they basically run on the same engine (yeah yeah they
renamed Gamebyro. You can call poop a diamond but that doesn't actually make it one)
3) Correct me if I'm wrong, but I think I recall seeing skeletons rise from the ground in Dawnguard. So they LITERALLY took the time to create this mechanic, then gave it to skeletons instead of the Volkihar.
All it needed was a very simple spawn-from-ground mechanic that makes them pop out of frozen lakes and perhaps from the walls of the frozen tundra to the northeast, aswell. That they can't pull you down under the lake? I'm pretty sure we're all logical enough to know that that'd make for an instant death, would be a complicated game mechanic aswell as a very boring one ("you walked over ice and are therefore dead!" Gee thanks game), thus we can settle for them popping out of the ice to ambush us.