Sorry about that.

Okay, after the battle with Calbrena and Golbez and the reunion with (SPOILER)Rydia, you end up in the throne room to talk with King Giott. After the discussion, before you leave the throne room, cast warp. You will be warped into the crystal room behind the throne room, and for one reason or another (probably because you were never meant to go back), the dark crystal is still there. Take it, and then leave the crystal room.

Now, when you go to the Sealed Cave, you have a dark crystal in your inventory, so when you get to the spot near the entrance where Kain (SPOILER)later betrays you, the scenario is set off due to the fact that you have a crystal with you. The scenario will go just as it did before with (SPOILER)Kain stealing the crystal and running off, without you ever having to set foot in the Sealed Cave.