Is there an unit/point ability that can be cast while walking ?
^ Sadly theres no warcraft based skill can cast a targeted unit/point without interrupting the current order.
Could be possible but needs a complex knowledge in triggering.
And you may need to set the cast point, blend time, castbacks animation of the caster to 0 value in its object data OR use the begins casting spell event and trigger the cooldown and its manacost.
Create custom channel ability based on a target point/unit, set its follow through time to 0, visible, order id carrion swarm, animation walk or stand.
You must use Unit Indexer System for saving the units' issued order for moving. ( save the location where the caster will move )
Unit starts the effect of an ability
Create an actual dummy or also trigger the whole spell event.
Its up to you if you want the dummy to cast a rain of fire, blizzard or carrion swarm on the spell targeted location. After that get the caster's user data and order the caster to move at the saved location. Done.
Im not sure if it will work but in this case the caster is still walking or moving to the issued move location without showing any spell animation order when he already casts the spell.
Or trigger the whole movement of the caster ( it will look like a little slide motion ) where you can use SetUnitX/Y to move the caster to the saved move location ( and does not interrupts the current order ) even channeling but its an odd btw.