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. All enemies have set HP and XP values. XP is awarded proportionally to damage dealt/total HP. Enemy HP stays the same on all difficulties.

The enemies are tougher but because of armor, not extra HP. And even if it did increase the HP, the XP value per enemy would still be the same.

Say you have an enemy with 100 HP, and its XP value is 1000. In that case, if a character does 20 points of damage to that enemy, they will receive 200 XP. That remains the same on all difficulties. But dealing that damage gets harder on harder difficulties because of extra enemy armor.

Even if that same enemy theoretically had 200 HP, all that would change would be the XP distribution. 25 points of damage would give only 125 experience. (25/200)*1000 instead of (25/100)*1000