The height and backstab advantages are borderline ridiculous in this game and needs to be re-evaluated.
And real life eludes many too. Do I not gain an advantage if I am on higher ground than you? Do I not gain an advantage when I come up behind you without you knowing I am behind you then stab or hit you? Do you not have a disadvantage when trying to hit someone on high ground? Do you not have a disadvantage due to your limited line of sight?
I could go on and on and on........
Point 1 - Having high ground would give you longer range and make it harder for the enemy to reach you, but wouldn't do much to increase your chance of hitting them except let you shoot over cover. Those are all advantages that don't have to break the game by being advantage, as in the mechanic that allows you to roll twice to hit and taking the greater number.
Point 2 - Yes, you would. That is why 5e has a stealth mechanic which allows you to hide as an action, or a bonus action as a rogue, and gain advantage on attacks against a target that doesn't know where you are.
Point 3/4 - There is already a 5e mechanic based on this. The target is either in 1/2 cover(half its body is obstructed from the attack) and gains a +2 to its AC and DEX saves, or 3/4 cover(3/4 of its body is obstructed from the attack) and gains +5 to its AC and DEX saves.
