During combat, once an opponent has acted, he is no longer flat-footed. There is no such thing as "behind" the enemy. Combat is constantly changing, combatants whirling around and such, therefore you cannot derive a clear front or back and that's why there is no such concept in D&D. Everyone has 360° action radius. That's what flanking is for to give you bonuses for surrounding an enemy in melee.
A 5'-step is just that, a 5'-step, a minimal movement to adjust your position in combat, while not taking an action to move. Tumbling is a way to negate the AoO provoked by moving. And yes, you can draw a weapon while tumbling, it's still movement.
Bye
Thanee