Does the DLC add mounted magic?

Can we go back to Skuldafn by dragonriding?

Can we shout/cast spells from a Dragon's back?

Are there Grahls? And Fryse Hags? And Berserkers? And snow wolves? And holy berries, wolfbane, belladona? And scribs/kwamas?

Are the Spriggans and Horkers from Solstheim different from the ones in Skyrim?

Is canis root of any help against werebears?

Yes, for mounted dragon combat.
No.
Yes.
No...
Some; One?
Maybe if you use it in a potion.

Can you waterwalk to Solstheim? Or do you have to go by boat?

Boat or Fast travel (in the north east of the Skyrim map, once you go to Solstheim the first time, there's an arrow on the minimap [It says "To Solstheim" when you cursor over it] that lets you fast travel there and its in the south west to go back to Skyrim [same as the other but it says "To Skyrim"])