The facing rules, DM and D&D in general doesn't consider that someone can freely and without any risks jump over the head of someone you're engaged with. It also don't consider that if you suceed this difficult movement, the creature won't turn and/or will totally forget you're presence.
In other words : You won't be considered "unseen". Even if you suceed such jump in D&D, you'll probably have an AOO, have to roll dice with really bad consequences if you fail and you also probably won't have the benefits of an invisibility potion of an attack when you're hidden or of an attack against an overwhelmed ennemy (surprise/flanking).
Nothing is wrong about getting advantages when your opponent is surprised/when you're unseen. Considering that you're unseen with this homebrew rule is only possible because TB is a static system in video game and at the moment, this rules totally exploit the TB system. This could work in P&P but only because there is a DM in real time.
Everything else you talked about are the rules of D&D and the RAW looks realistic in a story point of view without the need of a realtime DM. Considering that an attack in the back is always a surprise is not the same.
Just for the exemple, Solasta deal with this pretty well. If a creature see you, even if you walk in its back during your turn the creature know you're in it's back. If you want the advantage, you have to be out of its eye's field of view AND hide (potion, hide behind a wall, hide action, spell,...)