1. I hope you mean frost orb
you can frost bomb as fire as well which you should of course have
2. you're not only spreading dots on 2 adds. I think each time you infernoblast they go to a different add
3. glyphed combustion is always better when it stays on the mobs for the complete duration
4. we do focus an add, but any cleave damage is appreciated - you have to kill the healers first (at least in our strategy - we CC 2 Healers and 1 from the other 2 groups), that is what fire is really great at. Keep nuking that one healer while spreading your dots to the rest. Use Frost Bomb for maximum AoE damage.