You don't need the skull. (its part of a quest with qlaya) you only pick it up during the la tim quest. its not needed.

As for your problem Merci you need both Roca and clive in your party to talk to the guys in the bar after completing la tim, Clive should be in battle mode. You also again need both in your party when you go to the volcano. As for the volcano did you go to the right entrance?

Its the room at the top right when you enter.

Also when you do the Fred fight. Roca MUST be in battle mode with either la tim on her or in your inventory.