you could have a dummy caster cast frost nova on each unit in the aoe via a trigger. Or you could have a dummy caster cost frost nova on an enemy dummy target in the center of the blizzard target, setting the appropriate aoe.
To address the channeled part, i forget how it works.
im no expert but personally i think for something like that you are going to need triggers with variables. For example, you might want to use the channel ability as the dummy spell, and use a trigger to calculate the actual amount healed by using a formula like:
local integer HealAmount =...
if you go to youtube, type in "wc3 replacing weapons," by Roksinc. Has all the tools listed for download for removing weapons from unit models and is a good tutorial that shows an example of removing the footmans sword and replacing it with an axe.
Teleport to target, damaging all nearby units.
You have the Event and Condition right.
For Actions:
Set Point = (Position of (Target unit of ability being cast))
// good you made the variable, although you didnt use it completely
Change this line below
Unit - Move (triggering Unit)...