If I have a sprite on the Rockfall Path battlefield and hide a Mischievous Marai, on my next turn, can I unhide and play Mischievous Marai there, or if not, what happens?

No, you cannot play Mischievous Marai at Rockfall Path (or Rockfall Vale).

According to RiftJudge FAQ #3284 and #6500, playing a hidden unit at Rockfall Path is an illegal activation. Here is what happens:

1. Illegal Activation: When you attempt to play the hidden Mischievous Marai, the attempt is considered an illegal activation.
2. Rewind: The game state is rewound to before you attempted to play the unit.
3. Result: The card remains hidden facedown on Rockfall Path and cannot be played.
4. Consequence: Because it cannot be played, if you were to lose control of that battlefield (for example, if your Sprite token is removed and you have no other units there), the hidden Mischievous Marai would be sent to the trash.

As noted in the FAQ, hiding a unit at Rockfall Path is considered a poor strategic move because the game rules prevent units from being played to that location, even from a hidden state.