There is a section in the book that explicitly states that an attack that includes someone falling should not include additional falldamage, for pretty much the reasons you listed. The falling down part is then just part of the narrative, not part of the damage.
Actually having someone fall down should be a taken out result.
To move someone, you can simply have a number of shifts in the spell that move the target but don't do damage. A power:5 spell could move someone up to 5 zones away, minus borders of course. Remember here, that the game measures distance in zones, not feet. You can't deal stress that way. A weapon:8 spell that also moves the target for 8 shifts is way overpowered.
DoT spells have been discussed, and though I supported them before, I oppose them now. Again, it should be a narrative thing, not a mechanical thing. Dealing 8 shifts of stress in one exchange can easily be described as bouncing someone up and down, it just won't happen for multiple exchanges.