When you've destroyed the crystal and are teleported to the "endgame room" that just has the narrow walkway and the ghost, you can still use the forward/backward stones. They still make the sound effect and glow. I'm not actually sure if they have any kind of effect on the map. It seems like, while you're in this "place outside of time and space", that stones that try to affect time still work.
Ideally, it might make sense to make this room work specially: disable use of any item while there, and hide the hotbar entirely (you left your items behind in the "real world"). In fact, if we decide not to dump the player back into the original world (they have to acquire whatever they are going to BEFORE breaking the crystal, which is a reasonable thing many games require) we could just clear the inventory when we hide the hotbar.
When you've destroyed the crystal and are teleported to the "endgame room" that just has the narrow walkway and the ghost, you can still use the forward/backward stones. They still make the sound effect and glow. I'm not actually sure if they have any kind of effect on the map. It seems like, while you're in this "place outside of time and space", that stones that try to affect time still work.
Ideally, it might make sense to make this room work specially: disable use of any item while there, and hide the hotbar entirely (you left your items behind in the "real world"). In fact, if we decide not to dump the player back into the original world (they have to acquire whatever they are going to BEFORE breaking the crystal, which is a reasonable thing many games require) we could just clear the inventory when we hide the hotbar.