The backstab thing is another weird one really. I know it's "turn-based" but it's still supposed to reflect a fight happening, and no enemy would let you "circle them" to "backstab them". I much prefer having an ally close to the enemy giving you the ability to sneak attack, and that's it. Swashbucklers and Inquisitives are subclasses that can get sneak attack without having to have an ally close or having an advantage.
Exactly. The flanking rule implies a person cannot fully defend themselves if they are being assaulted from opposite sides. If only one person is attacking, surely you can turn with the person.
The 5e rule requires opposite sides for flanking but to make it simple in the game, just require two foes in melee range and both get advantage. It’s simple, tactical and more realistic and reduces the silly jumping while maintaining action economy.