Ok lets suppose I play through the entire Kingdom of Ehb map on EASY with a SOLO character and I kill every mob in the game. Then I do the exact same thing on HARD, will I be higher level at the end of the campaign on HARD mode?

Long story short -> No.

Harder dificulty only provides challenge. Mobs have more hp and your chars deal less dmg so you take more time to kill them but the xp they provide in both cases are exactly the same. There is literally no reason to play on hard dificulty unless you feel like you would like the mobs to have more hp and hit harder. In my case is a no go, game is long enough without unnecesarly long fights.

Difficulty increases enemy armor and damage. XP is awarded based on how much damage you deal. As the enemy HP stays the same on all difficulties, the XP is always the same.

The enemies are tougher but because of armor, not extra HP.