1) Serge is chosen because when he was dying from the Panther poison, his voice reached Schala and woke part of her, Balthazar saw this while studying the parallel time streams and it helped him be able to formulate his plan. Regretfully I can't remember who caused the storm but I do believe it was Schala actually, trying to reach Serge unconsciously.

2) FATE did this so it could bring Serge into Another World and gain the means to unlock the Frozen Flame from him, by switching bodies with him.

3. She went with he expedition that went to explore the Dead Sea and was frozen over in the eliminated time, with the rest of the Dragoons and General Viper. You see her among the group when you first enter the place.