Flame Strike does have a missile art, it's Blizzard I'm unsure about. Actually, I believe that almost every ability can have a missile art attached to it, you just need to define the speed and the model of the projectile. I know for certain, however, that Flame Strike has a missile art. If you pay close attention while using it, you will see a yellow-ish sphere head towards the target location. Just get rid of the part that appears instantly and it should work just how you want it to.
As for Cluster Rockets, it depends on the effect you want. If what you say about the Sphere ability is true, perhaps just give that ability to the caster unit?