How does copying work with Storm?
Neither of those events happen. Discounting the illegal interaction between Cloven Casting and Grapeshot, instead reference Twincast or Howl of the Horde.

Simplest path: you've cast 3 spells this turn, then cast Grapeshot. When you cast Grapeshot, you create copies of it for each spell you cast before it.

There are 4 objects named Grapeshot on the Stack, 3 copies and 1 represented by the card.

You then cast Twincast, targeting a copy of Grapeshot on the stack. You create an additional copy, but since you didn't cast this Grapeshot, the Storm effect doesn't Trigger.

5 objects named Grapeshot are on the stack, 4 copies and 1 represented by a card.