How difficult would you suppose Legendary would be if I were to have my level 81 character reset all skills by making them Legendary all at once?

Without exploits or mods?

Against Karstaag?

Since many dungeons have a level cap, do you suppose this mode would be more "realistic", or "Morrowind-istic"?

Lol i only knew about Karstag yesterday after looking up the giant corpse AFTER roaming the dungeons. And i imagine Legendary to be very hard, but its only as hard as you make it, as before you legendary your skills you could make potions and smith and enchant weapons and armour that would give you an edge in combat.

I'm hoping that at some poiint theres some food/drink/sleep requirement akin to fallout NV.

Your character would be reduced to the same level as a character running from Alduin in Helgen, but with more Health/Magicka/Stamina and better gear. Karstaag would just take a little longer to kill, because Many Perks have a very significant effect on the actual damage an attack does.