In answers to your questions:
1. I think you should put another Ball of Lightening and take out a scorpion, as you want to draw your balls of lightening as fast as possible, This will reduce your chance of a bad draw, by the way im presuming that the balls of lightening are going to be your main damager and scorpions are more of a finisher in case shields go up and only a few balls of lightening can do damage.
2. Yes momentum is the best choice as it costs the least and it gets through the shields which means if a fire shield comes up you wont lose all your creatures.
3. Only if you want EM's. Personally i would take out two scorpions and put two heals in.