You don't need to make contact, if the foot is above shoulder height, it's deemed as foul play.
No it's not. It's the same as overhead kicks (which would all be illegal under your interpretation of the law), it's about how safe it is in that moment to raise your foot (regardless of contact).
I've only seen one replay of it, but it seemed like the palace player was not in danger of getting hit in the head - he leaned in toward the ball and by the time his head got to where the leg was, the leg was on the way down. To me, that's safe enough.