npcs on high difficulty have more stamina or more armor?

You level faster when you do more hits,more dmg,or kill more npcs?

I mean - master leveling speed and adept leveling speed is same or not?

short answer yes

more health/armour means you have to bashs them more so combat skills will go up fast

Difficulty only effects Difficulty. Does not increase exp earned, better items, or faster skill levels.