But consider this...
Attacker well clear, only the full back to beat. Everyone else miles away and will play no part in this scenario. Defender attempts wrap tackle, turning the attacker, but attacker manages to go to ground. Defender by now is ahead of the ball, and facing his own posts. Attacker places ball a yard back from where he's lying - which is just beyond the defender's feet. Defender (who is behind the ball) kicks the ball.
In this scenario, PK against the defender. He played the ball while behind it, but not while behind the tackled player. So what could he have done?
Same scenario. Defender realises he has to get behind the tackled player as well as behind the ball before he can play it. He steps so that both feet are just behind the grounded tackled player, then steps forward and kicks the ball. Is this OK? He was always behind the ball, so that criterion is met. He also played the ball from directly behind the tackled player, so that criterion is met. He has complied fully with the law, without ever once stepping outside the tackle zone. QED.