The seminal point is:
Does the invocation in question can push targets once or multiple times?
Does it say push per cast or push per hit?
If its per hit, then per hit it is.
Thats my point of view on this matter of secondary arguments:
"Duration: Instantaneous" doesn't mean zero or no measureable time interval between cast-effect. It just means it doesnt last enough to be dispelled, but enough to be countered. The duration of the spell is small enough to be irrelevant in the round its being cast.
You can react, after all.
So, there IS some amount of time. The rules cover how much? No.
They need to cover it? Nope.
Magic missile is set apart of other spells by saying its hits are simultaneous.
The other spells say they are simultaneous?
No? Then they are not.
Consider a reaction, or ready action.
If X happens, then Y.
"If the warlock attacks me, i run around the corner, disappearing from view."
A) if the target allocation is defined on cast, then any change on this allocation is not possible. The target is chosen and be done with it.
B) If the target allocation is defined on each attack, then any subsequent changes can affect the choosing.
Then:
A) The warlock then (cast his spell, choose his target and only then...) attacks for the first time, and the target dives away from his vision, as the rightfull interruption of this ready action allows him to get full cover. But the target was already chosen for all attacks of his blasts, and all the blasts do... what? turn the corner on his tail? Follow him supernaturally tracking him behind the corner and hitting the wall?
B) The warlock cast his spell, choose his first target, make his first attack, only to see the rascal target disappear from view. He then chooses other targets for his remaining blasts, each one at his own time.
EDIT: the way i see it, B makes sense, A does not.
EDIT 2: Some musings:
Looks like there is foreign (meaning d&D game as "home") definitions of instantaneous involved. Magic the gathering (WoTC game too) uses those "target on cast" definitions, and ignore the "attack steps routine". Maybe there is some influence on this confusion?
