So you backstab someone that is you attack from the shadows an enmy on their back and you don't have a vantage? Seriously. Lets stay a bit away from the meticoluous argumentation based on rules (but more on personal perspective than objective reasoning), same goes for someone hitting from an higher place (that means they have an obvious advantage both because the bigger area covered by eyesight and difficulty for the attacker).
I'm going to break down what you wrote, and hopefully you'll understand why having faith in the rules of 5e is superior to Larian trying to force DoS mechanics where they don't belong.
1. If you attack "from the shadows", you are an "Unseen Attacker" in the rules of 5e. This is why someone who successfully uses Stealth gets Advantage on their attack. This is also why Rogues are allowed to attempt to Stealth as a Bonus Action in the rules of 5e, and every other class requires the use of a full Action. Advantage is *very* powerful, as I showed above, so this is why Rogue's are the only class that can attempt to stealth as a BA (in addition to other requirements within the rules that Larian has sort of implemented).
2. Hitting someone from a higher place can be more difficult as well because they can present a smaller profile (compare the human frame from level ground compared to directly above them). What you really mean to say, is that attacking someone from above means you get to ignore any potential Cover between you and your target...such as say another character between you and your target.
Did you actually read the post? I mean that, because if you did you will see that I provided answers to your questions as well as solutions you should agree with. Implementing Cover, or taking the easier route and offering the flat +2/-2 bonus to-hit, still provides you with a better chance to hit your target from Height and also for using tactical positioning.